The Project Gutenberg EBook of The National Geographic Magazine, Vol. I.,
No. 3, July, 1889, by Various
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Title: The National Geographic Magazine, Vol. I., No. 3, July, 1889
Author: Various
Release Date: November 4, 2015 [EBook #50383]
Language: English
*** START OF THIS PROJECT GUTENBERG EBOOK NATIONAL GEOGRAPHIC MAG., JULY 1889 ***
Produced by Ron Swanson
Vol. I. No. 3.
THE NATIONAL GEOGRAPHIC MAGAZINE.
PUBLISHED BY THE
NATIONAL GEOGRAPHIC SOCIETY.
WASHINGTON, D. C.
Price 50 Cents.
CONTENTS.
The Rivers and Valleys of Pennsylvania: William Morris Davis
(Illustrated by one map and twenty-five cuts.)
Topographic Models: Cosmos Mindeleff
(Illustrated by two plates.)
National Geographic Society--Abstract of Minutes
International Literary Contest to be held at Madrid, Spain
July, 1889.
PRESS OF TUTTLE, MOREHOUSE & TAYLOR, NEW HAVEN, CONN.
THE NATIONAL GEOGRAPHIC MAGAZINE.
Vol. I. 1889. No. 3.
THE RIVERS AND VALLEYS OF PENNSYLVANIA.[1]
BY WILLIAM MORRIS DAVIS.
"In Faltensystemen von sehr hohem Alter wurde die ursprüngliche
Anordnung der Langenthäler durch das Ueberhandnehmen der transversalen
Erosionsfurchen oft ganz und gar verwischt."
LÖWL. Petermann's Mittheilungen, xxviii, 1882, 411.
[Footnote 1: The substance of this essay was presented to the Society
in a lecture on February 8th, 1889, but since then it has been much
expanded.]
CONTENTS.
PART FIRST. _Introductory_.
1. Plan of work here proposed.
2. General description of the topography of Pennsylvania.
3. The drainage of Pennsylvania.
4. Previous studies of Appalachian drainage.
PART SECOND. _Outline of the geological history of the region_.
5. Conditions of formation.
6. Former extension of strata to the southeast.
7. Cambro-Silurian and Permian deformations.
8. Perm-Triassic denudation.
9. Newark deposition.
10. Jurassic tilting.
11. Jura-Cretaceous denudation.
12. Tertiary elevation and denudation.
13. Later changes of level.
14. Illustrations of Pennsylvanian topography.
PART THIRD. _General conception of the history of a river_.
15. The complete cycle of river life: youth, adolescence,
maturity and old age.
16. Mutual adjustment of river courses.
17. Terminology of rivers changed by adjustment.
18. Examples of adjustments.
19. Revival of rivers by elevation and drowning by depression.
20. Opportunity for new adjustments with revival.
21. Antecedent and superimposed rivers.
22. Simple, compound, composite and complex rivers.
PART FOURTH. _The development of the rivers of Pennsylvania_.
23. Means of distinguishing between antecedent and adjusted
consequent rivers.
24. Postulates of the argument.
25. Constructional Permian topography and consequent drainage.
26. The Jura mountains homologous with the Permian Alleghanies.
27. Development and adjustment of the Permian drainage.
28. Lateral water-gaps near the apex of synclinal ridges.
29. Departure of the Juniata from the Juniata-Catawissa syncline.
30. Avoidance of the Broad Top basin by the Juniata headwaters.
31. Reversal of larger rivers to southeast courses.
32. Capture of the Anthracite headwaters by the growing Susquehanna.
33. Present outward drainage of the Anthracite basins.
34. Homologies of the Susquehanna and Juniata.
35. Superimposition of the Susquehanna on two synclinal ridges.
36. Evidence of superimposition in the Susquehanna tributaries.
37. Events of the Tertiary cycle.
38. Tertiary adjustment of the Juniata on the Medina anticlines.
39. Migration of the Atlantic-Ohio divide.
40. Other examples of adjustments.
41. Events of the Quaternary cycle.
42. Doubtful cases.
43. Complicated history of our actual rivers.
44. Provisional conclusions.
PART FIRST. _Introductory_.
1. _Plan of work here proposed_.--No one now regards a river and its
valley as ready-made features of the earth's surface. All are convinced
that rivers have come to be what they are by slow processes of natural
development, in which every peculiarity of river-course and valley-form
has its appropriate cause. Being fully persuaded of the gradual and
systematic evolution of topographic forms, it is now desired, in
studying the rivers and valleys of Pennsylvania, to seek the causes of
the location of the streams in their present courses; to go back if
possible to the early date when central Pennsylvania was first raised
above the sea and trace the development of the several river systems
then implanted upon it from their ancient beginning to the present
time.
The existing topography and drainage system of the State will first be
briefly described. We must next inquire into the geological structure
of the region, follow at least in a general way the deformations and
changes of attitude and altitude that it has suffered, and consider the
amount of denudation that has been accomplished on its surface. We must
at the same time bear in mind the natural history of rivers, their
morphology and development; we must recognize the varying activities of
a river in its youth and old age, the adjustments of its adolescence
and maturity, and the revival of its decrepit powers when the land that
it drains is elevated and it enters a new cycle of life. Finally we
shall attempt to follow out the development of the rivers of
Pennsylvania by applying the general principles of river history to the
special case of Pennsylvania structure.
2. _General description of the topography of Pennsylvania_.--The
strongly marked topographic districts of Pennsylvania can hardly be
better described than by quoting the account given over a century ago
by Lewis Evans, of Philadelphia, in his "Analysis of a map of the
middle British colonies in America" (1755), which is as valuable from
its appreciative perception as it is interesting from its early date.
The following paragraphs are selected from his early pages:
"The land southwestward of Hudson's River is more regularly divided and
into a greater number of stages than the other. The first object worthy
of regard in this part is a rief or vein of rocks of the talky or
isinglassy kind, some two or three or half a dozen miles broad; rising
generally some small matter higher than the adjoining land; and
extending from New York city southwesterly by the lower falls of
Delaware, Schuylkill, Susquehanna, Gun-Powder, Patapsco, Potomack,
Rapahannock, James river and Ronoak. This was the antient maritime
boundary of America and forms a very regular curve. The land between
this rief and the sea and from the Navesink hills southwest ... may be
denominated the Lower Plains, and consists of soil washt down from
above and sand accumulated from the ocean. Where these plains are not
penetrated by rivers, they are a white sea-sand, about twenty feet deep
and perfectly barren, as no mixture of soil helps to enrich them. But
the borders of the rivers, which descend from the uplands, are rendered
fertile by the soil washt down with the floods and mixt with the sands
gathered from the sea. The substratum of sea-mud, shells and other
foreign subjects is a perfect confirmation of this supposition. And
hence it is that for 40 or 50 miles inland and all the way from the
Navesinks to Cape Florida, all is a perfect barren where the wash from
the uplands has not enriched the borders of the rivers; or some ponds
and defiles have not furnished proper support for the growth of white
cedars....
"From this rief of rocks, over which all the rivers fall, to that chain
of broken hills, called the South mountain, there is the distance of
50, 60 or 70 miles of very uneven ground, rising sensibly as you
advance further inland, and may be denominated the Upland. This
consists of veins of different kinds of soil and substrata some scores
of miles in length; and in some places overlaid with little ridges and
chains of hills. The declivity of the whole gives great rapidity to the
streams; and our violent gusts of rain have washt it all into gullies,
and carried down the soil to enrich the borders of the rivers in the
Lower Plains. These inequalities render half the country not easily
capable of culture, and impoverishes it, where torn up by the plow, by
daily washing away the richer mould that covers the surface.
"The South mountain is not in ridges like the Endless mountains, but in
small, broken, steep, stoney hills; nor does it run with so much
regularity. In some places it gradually degenerates to nothing, not to
appear again for some miles, and in others it spreads several miles in
breadth. Between South mountain and the hither chain of the Endless
mountains (often for distinction called the North mountain, and in some
places the Kittatinni and Pequélin), there is a valley of pretty even
good land, some 8, 10 or 20 miles wide, and is the most considerable
quantity of valuable land that the English are possest of; and runs
through New Jersey, Pensilvania, Mariland and Virginia. It has yet
obtained no general name, but may properly enough be called Piemont,
from its situation. Besides conveniences always attending good land,
this valley is everywhere enriched with Limestone.
"The Endless mountains, so called from a translation of the Indian name
bearing that signification, come next in order. They are not confusedly
scattered and in lofty peaks overtopping one another, but stretch in
long uniform ridges scarce half a mile perpendicular in any place above
the intermediate vallies. Their name is expressive of their extent,
though no doubt not in a literal sense.... The mountains are almost all
so many ridges with even tops and nearly of a height. To look from
these hills into the lower lands is but, as it were, into an ocean of
woods, swelled and deprest here and there by little inequalities, not
to be distinguished one part from another any more than the waves of
the real ocean. The uniformity of these mountains, though debarring us
of an advantage in this respect, makes some amends in another. They are
very regular in their courses, and confine the creeks and rivers that
run between; and if we know where the gaps are that let through these
streams, we are not at a loss to lay down their most considerable
inflections....
"To the northwestward of the Endless mountains is a country of vast
extent, and in a manner as high as the mountains themselves. To look at
the abrupt termination of it, near the sea level, as is the case on the
west side of Hudson's river below Albany, it looks as a vast high
mountain; for the Kaats Kills, though of more lofty stature than any
other mountains in these parts of America, are but the continuation of
the Plains on the top, and the cliffs of them in the front they present
towards Kinderhook. These Upper Plains are of extraordinary rich level
land, and extend from the Mohocks river through the country of the
Confederates.[2] Their termination northward is at a little distance
from Lake Ontario; but what it is westward is not known, for those most
extensive plains of Ohio are part of them."
[Footnote 2: Referring to the league of Indian tribes, so-called.]
These several districts recognized by Evans may be summarized as the
coastal plain, of nearly horizontal Cretaceous and later beds, just
entering the southeastern corner of Pennsylvania; the marginal upland
of contorted schists of disputed age; the South Mountain belt of
ancient and much disturbed crystalline rocks, commonly called Archean;
a space between these two traversed by the sandstone lowland of the
Newark formation;[3] the great Appalachian valley of crowded Cambrian
limestones and slates; the region of the even-crested, linear Paleozoic
ridges, bounded by Kittatinny or Blue mountain on the southeast and by
Alleghany mountain on the northwest, this being the area with which we
are here most concerned; and finally the Alleghany plateau, consisting
of nearly horizontal Devonian and Carboniferous beds and embracing all
the western part of the state. The whole region presents the most
emphatic expression not only of its structure but also of the more
recent cycles of development through which it has passed. Fig. 1
represents the stronger ridges and larger streams of the greater part
of the central district: it is reproduced from the expressive
Topographic Map of Pennsylvania (1871) by Lesley. The Susquehanna flows
down the middle, receiving the West Branch from Lock Haven and
Williamsport, the East Branch from Wilkes-Barre in the Wyoming basin,
and the Juniata from the Broad Top region, south of Huntingdon. The
Anthracite basins lie on the right, enclosed by zigzag ridges of Pocono
and Pottsville sandstone; the Plateau, trenched by the West Branch of
the Susquehanna is in the northwest. Medina sandstone forms most of the
central ridges.
[Footnote 3: Russell has lately recommended the revival of this term,
proposed many years ago by Redfield, as a non-committal name for the
"New red sandstones" of our Atlantic slope, commonly called Triassic.]
[Illustration: FIG. 1. Part of Topographic Map of Pennsylvania, by J.
P. Lesley (1871).]
3. _The drainage of Pennsylvania_.--The greater part of the Alleghany
plateau is drained westward into the Ohio, and with this we shall have
little to do. The remainder of the plateau drainage reaches the
Atlantic by two rivers, the Delaware and the Susquehanna, of which the
latter is the more special object of our study. The North and West
Branches of the Susquehanna rise in the plateau, which they traverse in
deep valleys; thence they enter the district of the central ranges,
where they unite and flow in broad lowlands among the even-crested
ridges. The Juniata brings the drainage of the Broad Top region to the
main stream just before their confluent current cuts across the
marginal Blue Mountain. The rock-rimmed basins of the anthracite region
are drained by small branches of the Susquehanna northward and
westward, and by the Schuylkill and Lehigh to the south and east. The
Delaware, which traverses the plateau between the Anthracite region and
the Catskill Mountain front, together with the Lehigh, the Schuylkill,
the little Swatara and the Susquehanna, cut the Blue Mountain by fine
water-gaps, and cross the great limestone valley. The Lehigh then turns
eastward and joins the Delaware, and the Swatara turns westward to the
Susquehanna; but the Delaware, Schuylkill and Susquehanna all continue
across South Mountain and the Newark belt, and into the low plateau of
schists beyond. The Schuylkill unites with the Delaware near
Philadelphia, just below the inner margin of the coastal plain; the
Delaware and the Susquehanna continue in their deflected estuaries to
the sea. All of these rivers and many of their side streams are at
present sunk in small valleys of moderate depth and width, below the
general surface of the lowlands, and are more or less complicated with
terrace gravels.
4. _Previous studies of Appalachian drainage_.--There have been no
special studies of the history of the rivers of Pennsylvania in the
light of what is now known of river development. A few recent essays of
rather general character as far as our rivers are concerned, may be
mentioned.
Peschel examined our rivers chiefly by means of general maps with
little regard to the structure and complicated history of the region.
He concluded that the several transverse rivers which break through the
mountains, namely, the Delaware, Susquehanna and Potomac, are guided by
fractures, anterior to the origin of the rivers.[4] There does not seem
to be sufficient evidence to support this obsolescent view, for most of
the water-gaps are located independently of fractures; nor can
Peschel's method of river study be trusted as leading to safe
conclusions.
[Footnote 4: Physische Erdkunde, 1880, ii, 442.]
Tietze regards our transverse valleys as antecedent;[5] but this was
made only as a general suggestion, for his examination of the structure
and development of the region is too brief to establish this and
exclude other views.
[Footnote 5: Jahrbuch Geol. Reichsanstalt, xxviii, 1878, 600.]
Löwl questions the conclusion reached by Tietze and ascribes the
transverse gaps to the backward or headwater erosion of external
streams, a process which he has done much to bring into its present
important position, and which for him replaces the persistence of
antecedent streams of other authors.[6]
[Footnote 6: Pet. Mitth., 1882, 405; Ueber Thalbildung, Prag, 1884.]
A brief article[7] that I wrote in comment on Löwl's first essay
several years ago now seems to me insufficient in its method. It
exaggerated the importance of antecedent streams; it took no sufficient
account of the several cycles of erosion through which the region has
certainly passed; and it neglected due consideration of the
readjustment of initial immature stream courses during more advanced
river-life. Since then, a few words in Löwl's essay have come to have
more and more significance to me; he says that in mountain systems of
very great age, the original arrangement of the longitudinal valleys
often becomes entirely confused by means of their conquest by
transverse erosion gaps. This suggestion has been so profitable to me
that I have placed the original sentence at the beginning of this
paper. Its thesis is the essential element of my present study.
[Footnote 7: Origin of Cross-valleys. Science, i, 1883, 325.]
Phillipson refers to the above-mentioned authors and gives a brief
account of the arrangement of drainage areas within our Appalachians,
but briefly dismisses the subject.[8] His essay contains a serviceable
bibliography.
[Footnote 8: Studien über Wasserscheiden. Leipsig, 1886, 149.]
If these several earlier essays have not reached any precise
conclusion, it may perhaps be because the details of the geological
structure and development of Pennsylvania have not been sufficiently
examined. Indeed, unless the reader has already become familiar with
the geological maps and reports of the Pennsylvania surveys and is
somewhat acquainted with its geography, I shall hardly hope to make my
case clear to him. The volumes that should be most carefully studied
are, first, the always inspiring classic, "Coal and its Topography"
(1856), by Lesley, in which the immediate relation of our topography to
the underlying structure is so finely described; the Geological Map of
Pennsylvania (1856), the result of the labors of the first survey of
the state; and the Geological Atlas of Counties, Volume X of the second
survey (1885). Besides these, the ponderous volumes of the final report
of the first survey and numerous reports on separate counties by the
second survey should be examined, as they contain many accounts of the
topography although saying very little about its development. If, in
addition to all this, the reader has seen the central district of the
state and marvelled at its even-crested, straight and zigzag ridges,
and walked through its narrow water-gaps into the enclosed coves that
they drain, he may then still better follow the considerations here
presented.
PART SECOND. _Outline of the geological history of the region_.
5. _Conditions of formation_.--The region in which the Susquehanna and
the neighboring rivers are now located is built in chief part of marine
sediments derived in paleozoic time from a large land area to the
southeast, whose northwest coast-line probably crossed Pennsylvania
somewhere in the southeastern part of the state; doubtless varying its
position, however, by many miles as the sea advanced and receded in
accordance with the changes in the relative altitudes of the land and
water surfaces, such as have been discussed by Newberry and Claypole.
The sediments thus accumulated are of enormous thickness, measuring
twenty or thirty thousand feet from their crystalline foundation to the
uppermost layer now remaining. The whole mass is essentially
conformable in the central part of the state. Some of the formations
are resistent, and these have determined the position of our ridges;
others are weaker and are chosen as the sites of valleys and lowlands.
The first are the Oneida and Medina sandstones, which will be here
generally referred to under the latter name alone, the Pocono sandstone
and the Pottsville conglomerate; to these may be added the fundamental
crystalline mass on which the whole series of bedded formations was
deposited, and the basal sandstone that is generally associated with
it. Wherever we now see these harder rocks, they rise above the
surrounding lowland surface. On the other hand, the weaker beds are the
Cambrian limestones (Trenton) and slates (Hudson River), all the
Silurian except the Medina above named, the whole of the Devonian--in
which however there are two hard beds of subordinate value, the
Oriskany sandstone and a Chemung sandstone and conglomerate, that form
low and broken ridges over the softer ground on either side of
them--and the Carboniferous (Mauch Chunk) red shales and some of the
weaker sandstones (Coal measures).
6. _Former extension of strata to the southeast_.--We are not much
concerned with the conditions under which this great series of beds was
formed; but, as will appear later, it is important for us to recognize
that the present southeastern margin of the beds is not by any means
their original margin in that direction. It is probable that the whole
mass of deposits, with greater or less variations of thickness,
extended at least twenty miles southeast of Blue Mountain, and that
many of the beds extended much farther. The reason for this conclusion
is a simple one. The several resistant beds above-mentioned consist of
quartz sand and pebbles that cannot be derived from the underlying beds
of limestones and shales; their only known source lay in the
crystalline rocks of the paleozoic land to the southeast. South
Mountain may possibly have made part of this paleozoic land; but it
seems more probable that it was land only during the earlier Archean
age, and that it was submerged and buried in Cambrian time and not
again brought to the light of day until it had been crushed into many
local anticlines[9] whose crests were uncovered by Permian and later
erosion. The occurrence of Cambrian limestone on either side of South
Mountain, taken with its compound anticlinal structure, makes it likely
that Medina time found this crystalline area entirely covered by the
Cambrian beds; Medina sands must therefore have come from farther still
to the southeast. A similar argument applies to the source of the
Pocono and Pottsville beds. The measure of twenty miles as the former
southeastern extension of the paleozoic formations therefore seems to
be a moderate one for the average of the whole series; perhaps forty
would be nearer the truth.
[Footnote 9: Lesley, as below.]
7. _Cambro-Silurian and Permian deformations_.--This great series of
once horizontal beds is now wonderfully distorted; but the distortions
follow a general rule of trending northeast and southwest, and of
diminishing in intensity from southeast to northwest. In the Hudson
Valley, it is well known that a considerable disturbance occurred
between Cambrian and Silurian time, for there the Medina lies
unconformably on the Hudson River shales. It seems likely, for reasons
that will be briefly given later on, that the same disturbance extended
into Pennsylvania and farther southwest, but that it affected only the
southeastern corner of the State; and that the unconformities in
evidence of it, which are preserved in the Hudson Valley, are here lost
by subsequent erosion. Waste of the ancient land and its
Cambro-Silurian annex still continued and furnished vast beds of
sandstone and sandy shales to the remaining marine area, until at last
the subsiding Paleozoic basin was filled up and the coal marshes
extended broadly across it. At this time we may picture the drainage of
the southeastern land area wandering rather slowly across the great
Carboniferous plains to the still submerged basin far to the west; a
condition of things that is not imperfectly represented, although in a
somewhat more advanced stage, by the existing drainage of the mountains
of the Carolinas across the more modern coastal plain to the Atlantic.
This condition was interrupted by the great Permian deformation that
gave rise to the main ranges of the Appalachians in Pennsylvania,
Virginia and Tennessee. The Permian name seems appropriate here, for
while the deformation may have begun at an earlier date, and may have
continued into Triassic time, its culmination seems to have been within
Permian limits. It was characterized by a resistless force of
compression, exerted in a southeast-northwest line, in obedience to
which the whole series of Paleozoic beds, even twenty or more thousand
feet in thickness, was crowded gradually into great and small folds,
trending northeast and southwest. The subjacent Archean terrane
doubtless shared more or less in the disturbance: for example, South
Mountain is described by Lesley as "not one mountain, but a system of
mountains separated by valleys. It is, geologically considered, a
system of anticlinals with troughs between.... It appears that the
South Mountain range ends eastward [in Cumberland and York Counties] in
a hand with five [anticlinal] fingers."[10]
[Footnote 10: Proc. Amer. Phil. Soc., xiii, 1873, 6.]
It may be concluded with fair probability that the folds began to rise
in the southeast, where they are crowded closest together, some of them
having begun here while coal marshes were still forming farther west;
and that the last folds to be begun were the fainter ones on the
plateau, now seen in Negro mountain and Chestnut and Laurel ridges. In
consequence of the inequalities in the force of compression or in the
resistance of the yielding mass, the folds do not continue indefinitely
with horizontal axes, but vary in height, rising or falling away in
great variety. Several adjacent folds often follow some general control
in this respect, their axes rising and falling together. It is to an
unequal yielding of this kind that we owe the location of the
Anthracite synclinal basins in eastern Pennsylvania, the Coal Measures
being now worn away from the prolongation of the synclines, which rise
in either direction.
8. _Perm-Triassic denudation_.--During and for a long time after this
period of mountain growth, the destructive processes of erosion wasted
the land and lowered its surface. An enormous amount of material was
thus swept away and laid down in some unknown ocean bed. We shall speak
of this as the Perm-Triassic period of erosion. A measure of its vast
accomplishment is seen when we find that the Newark formation, which is
generally correlated with Triassic or Jurassic time, lies unconformably
on the eroded surface of Cambrian and Archean rocks in the southeastern
part of the State, where we have concluded that the Paleozoic series
once existed; where the strata must have risen in a great mountain mass
as a result of the Appalachian deformations; and whence they must
therefore have been denuded before the deposition of the Newark beds.
Not only so; the moderate sinuosity of the southeastern or under
boundary of the Newark formation indicates clearly enough that the
surface on which that portion of the formation lies is one of no great
relief or inequality; and such a surface can be carved out of an
elevated land only after long continued denudation, by which
topographic development is carried beyond the time of its greatest
strength or maturity into the fainter expression of old age. This is a
matter of some importance in our study of the development of the rivers
of Pennsylvania; and it also constitutes a good part of the evidence
already referred to as indicating that there must have been some
earlier deformations of importance in the southeastern part of the
State; for it is hardly conceivable that the great Paleozoic mass could
have been so deeply worn off of the Newark belt between the making of
the last of the coal beds and the first of the Newark. It seems more in
accordance with the facts here recounted and with the teachings of
geological history in general to suppose, as we have here, that
something of the present deformation of the ancient rocks underlying
the Newark beds was given at an early date, such as that of the Green
Mountain growth; and that a certain amount of the erosion of the folded
beds was thus made possible in middle Paleozoic time; then again at
some later date, as Permian, a second period of mountain growth
arrived, and further folding was effected, and after this came deeper
erosion; thus dividing the destructive work that was done into several
parts, instead of crowding it all into the post-Carboniferous time
ordinarily assigned to it. It is indeed not impossible that an
important share of what we have called the Permian deformation was, as
above suggested, accomplished in the southeastern part of the State
while the coal beds were yet forming in the west; many grains of sand
in the sandstones of the Coal Measures may have had several temporary
halts in other sandstone beds between the time of their first erosion
from the Archean rocks and the much later time when they found the
resting place that they now occupy.[11]
[Footnote 11: These considerations may have value in showing that the
time in which the lateral crushing of the Appalachians was accomplished
was not so brief as is stated by Reade in a recent article in the
American Geologist, iii, 1889, 106.]
9. _Newark deposition_.--After the great Paleozoic and Perm-Triassic
erosions thus indicated, when the southeastern area of ancient
mountains had been well worn down and the Permian folds of the central
district had acquired a well developed drainage, there appeared an
opportunity for local deposition in the slow depression of a
northeast-southwest belt of the deeply wasted land, across the
southeastern part of the State; and into this trough-like depression,
the waste from the adjacent areas on either side was carried, building
the Newark formation. This may be referred to as the Newark or
Trias-Jurassic period of deposition. The volume of this formation is
unknown, as its thickness and original area are still undetermined; but
it is pretty surely of many thousand feet in vertical measure, and its
original area may have been easily a fifth or a quarter in excess of
its present area, if not larger yet. So great a local accumulation
seems to indicate that while the belt of deposition was sinking, the
adjacent areas were rising, in order to furnish a continual supply of
material; the occurrence of heavy conglomerates along the margins of
the Newark formation confirms this supposition, and the heavy breccias
near Reading indicate the occurrence of a strong topography and a
strong transporting agent to the northwest of this part of the Newark
belt. It will be necessary, when the development of the ancestors of
our present rivers is taken up, to consider the effects of the
depression that determined the locus of Newark deposition and of the
adjacent elevation that maintained a supply of material.
10. _Jurassic tilting_.--Newark deposition was stopped by a gradual
reversal of the conditions that introduced it. The depression of the
Newark belt was after a time reversed into elevation, accompanied by a
peculiar tilting, and again the waste of the region was carried away to
some unknown resting place. This disturbance, which may be regarded as
a revival of the Permian activity, culminated in Jurassic, or at least
in post-Newark time, and resulted in the production of the singular
monoclinal attitude of the formation; and as far as I can correlate it
with the accompanying change in the underlying structures, it involved
there an over-pushing of the closed folds of the Archean and Paleozoic
rocks. This is illustrated in figs. 2 and 3, in which the original and
disturbed attitudes of the Newark and the underlying formations are
roughly shown, the over-pushing of the fundamental folds causing the
monoclinal and probably faulted structure in the overlying beds.[12] If
this be true, we might suspect that the unsymmetrical attitude of the
Appalachian folds, noted by Rogers as a characteristic of the range, is
a feature that was intensified if not originated in Jurassic and not in
Permian time.
[Footnote 12: Amer. Journ. Science, xxxii, 1886, 342; and Seventh Ann.
Rept. U. S. Geol. Survey, 1888, 486.]
[Illustration: FIG. 2.]
[Illustration: FIG. 3.]
It is not to be supposed that the Jurassic deformation was limited to
the area of the Newark beds; it may have extended some way on either
side; but it presumably faded out at no great distance, for it has not
been detected in the history of the Atlantic and Mississippi regions
remote from the Newark belt. In the district of the central folds of
Pennsylvania, with which we are particularly concerned, this
deformation was probably expressed in a further folding and
over-pushing of the already partly folded beds, with rapidly decreasing
effect to the northwest; and perhaps also by slip-faults, which at the
surface of the ground nearly followed the bedding planes: but this is
evidently hypothetical to a high degree. The essential point for our
subsequent consideration is that the Jurassic deformation was probably
accompanied by a moderate elevation, for it allowed the erosion of the
Newark beds and of laterally adjacent areas as well.
11. _Jura-Cretaceous denudation_.--In consequence of this elevation, a
new cycle of erosion was entered upon, which I shall call the
Jura-Cretaceous cycle. It allowed the accomplishment of a vast work,
which ended in the production of a general lowland of denudation, a
wide area of faint relief, whose elevated remnants are now to be seen
in the even ridge-crests that so strongly characterize the central
district, as well as in certain other even uplands, now etched by the
erosion of a later cycle of destructive work. I shall not here take
space for the deliberate statement of the argument leading to this end,
but its elements are as follows: the extraordinarily persistent
accordance among the crest-line altitudes of many Medina and
Carboniferous ridges in the central district; the generally
corresponding elevation of the western plateau surface, itself a
surface of erosion, but now trenched by relatively deep and narrow
valleys; the generally uniform and consistent altitude of the uplands
in the crystalline highlands of northern New Jersey and in the South
Mountains of Pennsylvania; and the extension of the same general
surface, descending slowly eastward, over the even crest-lines of the
Newark trap ridges. Besides the evidence of less continental elevation
thus deduced from the topography, it may be noted that a lower stand of
the land in Cretaceous time than now is indicated by the erosion that
the Cretaceous beds have suffered in consequence of the elevation that
followed their deposition. The Cretaceous transgression in the western
states doubtless bears on the problem also. Finally it may be fairly
urged that it is more accordant with what is known about old mountains
in general to suppose that their mass has stood at different attitudes
with respect to base level during their long period of denudation than
to suppose that they have held one attitude through all the time since
their deformation.
It is natural enough that the former maintenance of some lower altitude
than the present should have expression in the form of the country, if
not now extinguished by subsequent erosion. It is simply the reverse of
this statement that leads us to the above-stated conclusion. We may be
sure that the long maintained period of relative quiet was of great
importance in allowing time for the mature adjustment of the rivers of
the region, and hence due account must be taken of it in a later
section. I say relative quiet, for there were certainly subordinate
oscillations of greater or less value; McGee has detected records of
one of these about the beginning of Cretaceous time, but its effects
are not now known to be of geographic value; that is, they do not now
manifest themselves in the form of the present surface of the land, but
only in the manner of deposition and ancient erosion of certain
deposits.[13] Another subordinate oscillation in the sense of a
moderate depression seems to have extended through middle and later
Cretaceous time, resulting in an inland transgression of the sea and
the deposit of the Cretaceous formation unconformably on the previous
land surface for a considerable distance beyond the present margin of
the formation.[14] This is important as affecting our rivers. Although
these oscillations were of considerable geological value, I do not
think that for the present purposes they call for any primary division
of the Jura-Cretaceous cycle; for as the result of this long period of
denudation we find but a single record in the great lowland of erosion
above described, a record of prime importance in the geographic
development of our region, that will often be referred to. The surface
of faint relief then completed may be called the Cretaceous baselevel
lowland. It may be pictured as a low, undulating plain of wide extent,
with a portion of its Atlantic margin submerged and covered over with a
relatively thin marine deposit of sands, marls and clays.
[Footnote 13: Amer. Jour. Science, xxxv, 1888, 367, 448.]
[Footnote 14: This statement is based on a study of the geographic
evolution of northern New Jersey, in preparation for publication.]
12. _Tertiary elevation and denudation_.--This broad lowland is a
lowland no longer. It has been raised over the greater part of its area
into a highland, with an elevation of from one to three thousand feet,
sloping gently eastward and descending under the Atlantic level near
the present margin of the Cretaceous formation. The elevation seems to
have taken place early in Tertiary time, and will be referred to as of
that date. Opportunity was then given for the revival of the previously
exhausted forces of denudation, and as a consequence we now see the
formerly even surface of the plain greatly roughened by the incision of
deep valleys and the opening of broad lowlands on its softer rocks.
Only the harder rocks retain indications of the even surface which once
stretched continuously across the whole area. The best indication of
the average altitude at which the mass stood through the greater part
of post-Cretaceous time is to be found on the weak shales of the Newark
formation in New Jersey and Pennsylvania, and on the weak Cambrian
limestones of the great Kittatinny valley; for both of these areas have
been actually almost baselevelled again in the Tertiary cycle. They
will be referred to as the Tertiary baselevel lowlands; and the valleys
corresponding to them, cut in the harder rocks, as well as the rolling
lowlands between the ridges of the central district of Pennsylvania
will be regarded as of the same date. Whatever variations of level
occurred in this cycle of development do not seem to have left marks of
importance on the inland surface, though they may have had greater
significance near the coast.
13. _Later changes of level_.--Again at the close of Tertiary time,
there was an elevation of moderate amount, and to this may be referred
the trenches that are so distinctly cut across the Tertiary baselevel
lowland by the larger rivers, as well as the lateral shallower channels
of the smaller streams. This will be called the Quaternary cycle; and
for the present no further mention of the oscillations known to have
occurred in this division of time need be considered; the reader may
find careful discussion of them in the paper by McGee, above referred
to. It is proper that I should add that the suggestion of baselevelling
both of the crest-lines and of the lowlands, that I have found so
profitable in this and other work, is due largely to personal
conference with Messrs. Gilbert and McGee of the Geological Survey; but
it is not desired to make them in any way responsible for the
statements here given.
[Illustration: FIG. 4.]
[Illustration: FIG. 5.]
14. _Illustrations of Pennsylvanian topography_.--A few sketches made
during a recent recess-trip with several students through Pennsylvania
may be introduced in this connection. The first, fig. 4, is a view from
Jenny Jump mountain, on the northwestern side of the New Jersey
highlands, looking northwest across the Kittatinny valley-lowland to
Blue or Kittatinny mountain, where it is cut at the Delaware Water-gap.
The extraordinarily level crest of the mountain preserves record of the
Cretaceous baselevel lowland; since the elevation of this ancient
lowland, its softer rocks have, as it were, been etched out, leaving
the harder ones in relief; thus the present valley-lowland is to be
explained. In consequence of the still later elevation of less amount,
the Delaware has cut a trench in the present lowland, which is partly
seen to the left in the sketch. Fig. 5 is a general view of the Lehigh
plateau and cañon, looking south from Bald Mountain just above Penn
Haven Junction. Blue mountain is the most distant crest, seen for a
little space. The ridges near and above Mauch Chunk form the other
outlines; all rising to an astonishingly even altitude, in spite of
their great diversity of structure. Before the existing valleys were
excavated, the upland surface must have been an even plain--the
Cretaceous baselevel lowland elevated into a plateau. The valleys cut
into the plateau during the Tertiary cycle are narrow here, because the
rocks are mostly hard. The steep slopes of the cañon-like valley of the
Lehigh and the even crests of the ridges manifestly belong to different
cycles of development. Figs. 6 and 7 are gaps cut in Black Log and
Shade mountain, by a small upper branch stream of the Juniata in
southeastern Huntingdon county. The stream traverses a breached
anticlinal of Medina sandstone, of which these mountains are the
lateral members. A long narrow valley is opened on the axial Trenton
limestone between the two. The gaps are not opposite to each other, and
therefore in looking through either gap from the outer country the even
crest of the further ridge is seen beyond the axial valley. The gap in
Black Log mountain, fig. 6, is located on a small fracture, but in this
respect it is unlike most of its fellows.[15] The striking similarity
of the two views illustrates the uniformity that so strongly
characterizes the Medina ridges of the central district. Fig. 8 is in
good part an ideal view, based on sketches on the upper Susquehanna,
and designed to present a typical illustration of the more significant
features of the region. It shows the even crest-lines of a high Medina
or Pocono ridge in the background, retaining the form given to it in
the Cretaceous cycle; the even lowlands in the foreground, opened on
the weaker Siluro-Devonian rocks in the Tertiary cycle; and the uneven
ridges in the middle distance marking the Oriskany and Chemung beds of
intermediate hardness that have lost the Cretaceous level and yet have
not been reduced to the Tertiary lowland. The Susquehanna flows
distinctly below the lowland plain, and the small side streams run in
narrow trenches of late Tertiary and Quaternary date.
[Footnote 15: Second Geol. Surv. Pa., Report T_{3}, 19.]
[Illustration: FIG. 6.]
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
If this interpretation is accepted, and the Permian mountains are seen
to have been once greatly reduced and at a later time worn out, while
the ridges of to-day are merely the relief left by the etching of
Tertiary valleys in a Cretaceous baselevelled lowland, then we may well
conclude with Powell that "mountains cannot remain long as mountains;
they are ephemeral topographic forms."[16]
[Footnote 16: Geol. Uinta Mountains, 1876, 196.]
PART THIRD. _General conception of the history of a river_.
15. _The complete cycle of river life: youth, adolescence, maturity and
old age_.--The general outline of an ideal river's history may be now
considered, preparatory to examining the special history of the rivers
of Pennsylvania, as controlled by the geological events just narrated.
Rivers are so long lived and survive with more or less modification so
many changes in the attitude and even in the structure of the land,
that the best way of entering on their discussion seems to be to
examine the development of an ideal river of simple history, and from
the general features thus discovered, it may then be possible to
unravel the complex sequence of events that leads to the present
condition of actual rivers of complicated history.
A river that is established on a new land may be called an original
river. It must at first be of the kind known as a consequent river, for
it has no ancestor from which to be derived. Examples of simple
original rivers may be seen in young plains, of which southern New
Jersey furnishes a fair illustration. Examples of essentially original
rivers may be seen also in regions of recent and rapid displacement,
such as the Jura or the broken country of southern Idaho, where the
directly consequent character of the drainage leads us to conclude
that, if any rivers occupied these regions before their recent
deformation, they were so completely extinguished by the newly made
slopes that we see nothing of them now.
Once established, an original river advances through its long life,
manifesting certain peculiarities of youth, maturity and old age, by
which its successive stages of growth may be recognized without much
difficulty. For the sake of simplicity, let us suppose the land mass,
on which an original river has begun its work, stands perfectly still
after its first elevation or deformation, and so remains until the
river has completed its task of carrying away all the mass of rocks
that rise above its baselevel. This lapse of time will be called a
cycle in the life of a river. A complete cycle is a long measure of
time in regions of great elevation or of hard rocks; but whether or not
any river ever passed through a single cycle of life without
interruption we need not now inquire. Our purpose is only to learn what
changes it would experience if it did thus develop steadily from
infancy to old age without disturbance.
In its infancy, the river drains its basin imperfectly; for it is then
embarrassed by the original inequalities of the surface, and lakes
collect in all the depressions. At such time, the ratio of evaporation
to rainfall is relatively large, and the ratio of transported land
waste to rainfall is small. The channels followed by the streams that
compose the river as a whole are narrow and shallow, and their number
is small compared to that which will be developed at a later stage. The
divides by which the side-streams are separated are poorly marked, and
in level countries are surfaces of considerable area and not lines at
all. It is only in the later maturity of a system that the divides are
reduced to lines by the consumption of the softer rocks on either side.
The difference between constructional forms and those forms that are
due to the action of denuding forces is in a general way so easily
recognized, that immaturity and maturity of a drainage area can be
readily discriminated. In the truly infantile drainage system of the
Red River of the North, the inter-stream areas are so absolutely flat
that water collects on them in wet weather, not having either original
structural slope or subsequently developed denuded slope to lead it to
the streams. On the almost equally young lava blocks of southern
Oregon, the well-marked slopes are as yet hardly channeled by the flow
of rain down them, and the depressions among the tilted blocks are
still undrained, unfilled basins.
As the river becomes adolescent, its channels are deepened and all the
larger ones descend close to baselevel. If local contrasts of hardness
allow a quick deepening of the down-stream part of the channel, while
the part next up-stream resists erosion, a cascade or waterfall
results; but like the lakes of earlier youth, it is evanescent, and
endures but a small part of the whole cycle of growth; but the falls on
the small headwater streams of a large river may last into its
maturity, just as there are young twigs on the branches of a large
tree. With the deepening of the channels, there comes an increase in
the number of gulleys on the slopes of the channel; the gulleys grow
into ravines and these into side valleys, joining their master streams
at right angles (La Noë and Margerie). With their continued
development, the maturity of the system is reached; it is marked by an
almost complete acquisition of every part of the original
constructional surface by erosion under the guidance of the streams, so
that every drop of rain that falls finds a way prepared to lead it to a
stream and then to the ocean, its goal. The lakes of initial
imperfection have long since disappeared; the waterfalls of adolescence
have been worn back, unless on the still young headwaters. With the
increase of the number of side-streams, ramifying into all parts of the
drainage basin, there is a proportionate increase in the surface of the
valley slopes, and with this comes an increase in the rate of waste
under atmospheric forces; hence it is at maturity that the river
receives and carries the greatest load; indeed, the increase may be
carried so far that the lower trunk-stream, of gentle slope in its
early maturity, is unable to carry the load brought to it by the upper
branches, and therefore resorts to the temporary expedient of laying it
aside in a flood-plain. The level of the flood-plain is sometimes built
up faster than the small side-streams of the lower course can fill
their valleys, and hence they are converted for a little distance above
their mouths into shallow lakes. The growth of the flood-plain also
results in carrying the point of junction of tributaries farther and
farther down stream, and at last in turning lateral streams aside from
the main stream, sometimes forcing them to follow independent courses
to the sea (Lombardini). But although thus separated from the main
trunk, it would be no more rational to regard such streams as
independent rivers than it would be to regard the branch of an old
tree, now fallen to the ground in the decay of advancing age, as an
independent plant; both are detached portions of a single individual,
from which they have been separated in the normal processes of growth
and decay.
In the later and quieter old age of a river system, the waste of the
land is yielded slower by reason of the diminishing slopes of the
valley sides; then the headwater streams deliver less detritus to the
main channel, which, thus relieved, turns to its postponed task of
carrying its former excess of load to the sea, and cuts terraces in its
flood-plain, preparatory to sweeping it away. It does not always find
the buried channel again, and perhaps settling down on a low spur a
little to one side of its old line, produces a rapid or a low fall on
the lower slope of such an obstruction (Penck). Such courses may be
called locally superimposed.
It is only during maturity and for a time before and afterwards that
the three divisions of a river, commonly recognized, appear most
distinctly; the torrent portion being the still young headwater
branches, growing by gnawing backwards at their sources; the valley
portion proper, where longer time of work has enabled the valley to
obtain a greater depth and width; and the lower flood-plain portion,
where the temporary deposition of the excess of load is made until the
activity of middle life is past.
Maturity seems to be a proper term to apply to this long enduring
stage; for as in organic forms, where the term first came into use, it
here also signifies the highest development of all functions between a
youth of endeavor towards better work and an old age of relinquishment
of fullest powers. It is the mature river in which the rainfall is best
lead away to the sea, and which carries with it the greatest load of
land waste; it is at maturity that the regular descent and steady flow
of the river is best developed, being the least delayed in lakes and
least overhurried in impetuous falls.
Maturity past, and the power of the river is on the decay. The relief
of the land diminishes, for the streams no longer deepen their valleys
although the hill tops are degraded; and with the general loss of
elevation, there is a failure of rainfall to a certain extent; for it
is well known that up to certain considerable altitudes rainfall
increases with height. A hyetographic and a hypsometric map of a
country for this reason show a marked correspondence. The slopes of the
headwaters decrease and the valley sides widen so far that the land
waste descends from them slower than before. Later, what with failure
of rainfall and decrease of slope, there is perhaps a return to the
early imperfection of drainage, and the number of side streams
diminishes as branches fall from a dying tree. The flood-plains of
maturity are carried down to the sea, and at last the river settles
down to an old age of well-earned rest with gentle flow and light load,
little work remaining to be done. The great task that the river entered
upon is completed.
16. _Mutual adjustment of river courses_.--In certain structures,
chiefly those of mountainous disorder on which the streams are at first
high above baselevel, there is a process of adjustment extremely
characteristic of quiet river development, by which the down-hill
courses that were chosen in early life, and as we may say unadvisedly
and with the heedlessness and little foresight of youth, are given up
for others better fitted for the work of the mature river system. A
change of this kind happens when the young stream taking the lowest
line for its guide happens to flow on a hard bed at a considerable
height above baselevel, while its branches on one side or the other
have opened channels on softer beds: a part of the main channel may
then be deserted by the withdrawal of its upper waters to a lower
course by way of a side stream. The change to better adjustment also
happens when the initial course of the main stream is much longer than
a course that may be offered to its upper portion by the backward
gnawing of an adjacent stream (Löwl, Penck). Sometimes the lateral
cutting or planation that characterizes the main trunk of a mature
river gives it possession of an adjacent smaller stream whose bed is at
a higher level (Gilbert). A general account of these processes may be
found in Phillippson's serviceable "Studien über Wasserscheiden"
(Leipzig, 1886). This whole matter is of much importance and deserves
deliberate examination. It should be remembered that changes in river
courses of the kind now referred to are unconnected with any external
disturbance of the river basin, and are purely normal spontaneous acts
during advancing development. Two examples, pertinent to our special
study, will be considered.
[Illustration: FIG. 9.]
[Illustration: FIG. 10.]
Let AB, fig. 9, be a stream whose initial consequent course led it down
the gently sloping axial trough of a syncline. The constructional
surface of the syncline is shown by contours. Let the succession of
beds to be discovered by erosion be indicated in a section, laid in
proper position on the several diagrams, but revolved into the
horizontal plane, the harder beds being dotted and the baselevel
standing at OO. Small side streams will soon be developed on the slopes
of the syncline, in positions determined by cross-fractures or more
often by what we call accident; the action of streams in similar
synclines on the outside of the enclosing anticlines will be omitted
for the sake of simplicity. In time, the side streams will cut through
the harder upper bed M and enter the softer bed N, on which
longitudinal channels, indicated by hachures, will be extended along
the strike, fig. 10 (La Noë and Margerie). Let these be called
"subsequent" streams. Consider two side streams of this kind, C and D,
heading against each other at E, one joining the main stream lower down
the axis of the syncline than the other. The headwaters of C will rob
the headwaters of D, because the deepening of the channel of D is
retarded by its having to join the main stream at a point where the
hard bed in the axis of the fold holds the main channel well above
baselevel. The notch cut by D will then be changed from a water-gap to
a wind-gap and the upper portion of D will find exit through the notch
cut by C, as in fig. 11. As other subsequent headwaters make capture of
C, the greater depth to which the lateral valley is cut on the soft
rock causes a slow migration of the divides in the abandoned gaps
towards the main stream, and before long the upper part of the main
stream itself will be led out of the synclinal axis to follow the
monoclinal valley at one side for a distance, fig. 12, until the axis
can be rejoined through the gap where the axial portion of the
controlling hard bed is near or at baselevel. The upper part of the
synclinal trough will then be attacked by undercutting on the slope of
the quickly deepened channels of the lateral streams, and the hard bed
will be worn away in the higher part of the axis before it is consumed
in the lower part. The location of the successful lateral stream on one
or the other side of the syncline may be determined by the dip of the
beds, gaps being cut quicker on steep than on gentle dips. If another
hard bed is encountered below the soft one, the process will be
repeated; and the mature arrangement of the streams will be as in fig.
13 (on a smaller scale than the preceding), running obliquely off the
axis of the fold where a hard bed of the syncline rises above
baselevel, and returning to the axis where the hard bed is below or at
baselevel; a monoclinal stream wandering gradually from the axis along
the strike of the soft bed, AE, by which the side-valley is located and
returning abruptly to the axis by a cataclinal[17] stream in a
transverse gap, EB, in the next higher hard bed, and there rejoining
the diminished representative or survivor of the original axial or
synclinal stream, GB.
[Footnote 17: See the terminology suggested by Powell. Expl. Col. R. of
the West, 1875, 160. This terminology is applicable only to the most
detailed study of our rivers, by reason of their crossing so many
folds, and changing so often from longitudinal to transverse courses.]
[Illustration: FIG. 11.]
[Illustration: FIG. 12.]
[Illustration: FIG. 13.]
17. _Terminology of rivers changed by adjustment_.--A special
terminology is needed for easy reference to the several parts of the
streams concerned in such an adjustment. Let AB and CD, fig. 14, be
streams of unequal size cutting gaps, H and G, in a ridge that lies
transverse to their course. CD being larger than AB will deepen its gap
faster. Of two subsequent streams, JE and JF, growing on the up-stream
side of the ridge, JE will have the steeper slope, because it joins the
deeper master-stream. The divide, J, will therefore be driven towards
AB, and if all the conditions concerned conspire favorably, JE will at
last tap AB at F, and lead the upper part, AF, out by the line FEGD,
fig. 15, through the deeper gap, G. We may then say that JE becomes the
_divertor_ of AF, which is _diverted_; and when the process is
completed, by the transfer of the divide from J, on the soft rocks, to
a stable location, H, on the hard rocks, there will be a short
_inverted_ stream, HF; while HB is the remaining _beheaded_ portion of
the original stream, AB, and the water-gap of AB becomes a wind-gap, H.
It is very desirable that geographic exploration should discover
examples of the process of adjustment in its several stages. The
preparatory stage is easily recognized by the difference in the size of
the two main streams, the difference in the depth of their gaps, and
the unsymmetrical position of the divide, J. The very brief stage of
transition gives us the rare examples of bifurcating streams. For a
short time after capture of the diverted stream by the divertor, the
new divide will lie between F and H, in an unstable position, the
duration of this time depending on the energy of the process of
capture.
[Illustration: FIG. 14.]
[Illustration: FIG. 15.]
The consequences resulting from readjustments of this kind by which
their recent occurrence can be detected are: a relatively sudden
increase of volume of the divertor and hence a rapid deepening of the
course of the diverting stream, FE, and of the diverted, AF, near the
point of capture; small side-streams of these two being unable to keep
pace with this change will join their masters in local rapids, which
work up stream gradually and fade away (Löwl, Penck, McGee). The
expanded portion, ED, of the larger stream, CD, already of faint slope,
may be locally overcome for a time with the increase of detritus that
will be thus delivered to it at the entrance, E, of the divertor; while
the beheaded stream, HB, will find itself embarrassed to live up to the
habits of its large valley [Heim]. Geographic exploration with these
matters in mind offers opportunity for the most attractive discoveries.
[Illustration: FIG. 16.]
[Illustration: FIG. 17.]
[Illustration: FIG. 18.]
18. _Examples of adjustment_.--Another case is roughly figured in the
next three diagrams, figs. 16, 17, 18. Two adjacent synclinal streams,
EA and HB, join a transverse master stream, C, but the synclines are of
different forms; the surface axis of one, EA, stands at some altitude
above baselevel until it nearly reaches the place of the transverse
stream; while the axis of the other, HB, descends near baselevel at a
considerable distance from the transverse stream. As lateral valleys, E
and D, are opened on the anticline between the synclines by a process
similar to that already described, the divide separating them will
shift towards the stream of fainter slope, that is, towards the
syncline, EA, whose axis holds its hard beds above baselevel; and in
time the upper part of the main stream will be withdrawn from this
syncline to follow an easier course by crossing to the other, as in
fig. 17. If the elevation of the synclinal axis, AES, take the shape of
a long flat arch, descending at the further end into a synclinal lake
basin, S, whose outlet is along the arching axis, SA, then the mature
arrangement of stream courses will lead the lake outlet away from the
axis by some gap in the nearer ascending part of the arch where the
controlling hard bed falls near to baselevel, as at F, fig. 18,[18] and
will take it by some subsequent course, FD, across the lowland that is
opened on the soft beds between the synclines, and carry it into the
lower syncline, HB, at D where the hard beds descend below baselevel.
[Footnote 18: This figure would be improved if a greater amount of
wasting around the margin of the hard bed were indicated in comparison
with the preceding figure.]
The variety of adjustments following the general principle here
indicated is infinite. Changes of greater or less value are thus
introduced in the initial drainage areas, until, after attaining an
attitude of equilibrium, further change is arrested, or if occurring,
is relatively insignificant. It should be noticed that the new stream
courses thus chosen are not named by any of the terms now current to
express the relation of stream and land history; they are neither
consequent, antecedent nor superimposed. The stream is truly still an
original stream, although no longer young; but its channel is not in
all parts strictly consequent on the initial constructional form of the
land that it drains. Streams thus re-arranged may therefore be named
original streams of mature adjustment.
It should be clearly recognized that the process of adjustment is a
very slow one, unless measured in the extremely long units of a river's
life. It progresses no faster than the weathering away of the slopes of
a divide, and here as a rule weathering is deliberate to say the least,
unless accelerated by a fortunate combination of favoring conditions.
Among these conditions, great altitude of the mass exposed to erosion
stands first, and deep channeling of streams below the surface--that
is, the adolescent stage of drainage development--stands second. The
opportunity for the lateral migration of a divide will depend on the
inequality of the slopes on its two sides, and here the most important
factors are length of the two opposite stream courses from the water
parting to the common baselevel of the two, and inequality of structure
by which one stream may have an easy course and the other a hard one.
It is manifest that all these conditions for active shifting of divides
are best united in young and high mountain ranges, and hence it is that
river adjustments have been found and studied more in the Alps than
elsewhere.
19. _Revival of rivers by elevation and drowning by depression_.--I
make no contention that any river in the world ever passed through a
simple uninterrupted cycle of the orderly kind here described. But by
examining many rivers, some young and some old, I do not doubt that
this portrayal of the ideal would be found to be fairly correct if
opportunity were offered for its development. The intention of the
sketch is simply to prepare the way for the better understanding of our
actual rivers of more complicated history.
At the close or at any time during the passage of an initial cycle such
as the one just considered, the drainage area of a river system may be
bodily elevated. The river is then turned back to a new youth and
enters a new cycle of development. This is an extremely common
occurrence with rivers, whose life is so long that they commonly
outlive the duration of a quiescent stage in the history of the land.
Such rivers may be called revived. Examples may be given in which
streams are now in their second or third period of revival, the
elevations that separate their cycles following so soon that but little
work was accomplished in the quiescent intervals.
The antithesis of this is the effect of depression, by which the lower
course may be drowned, flooded or fjorded. This change is, if slow,
favorable to the development of flood-plains in the lower course; but
it is not essential to their production. If the change is more rapid,
open estuaries are formed, to be transformed to delta-lowlands later
on.
20. _Opportunity for new adjustments with revival_.--One of the most
common effects of the revival of a river by general elevation is a new
adjustment of its course to a greater or less extent, as a result of
the new relation of baselevel to the hard and soft beds on which the
streams had adjusted themselves in the previous cycle. Synclinal
mountains are most easily explained as results of drainage changes of
this kind [Science, Dec. 21st, 1888]. Streams thus rearranged may be
said to be adjusted through elevation or revival. It is to be hoped
that, as our study advances, single names of brief and appropriate form
may replace these paraphrases; but at present it seems advisable to
keep the desired idea before the mind by a descriptive phrase, even at
the sacrifice of brevity. A significant example may be described.
[Illustration: FIG. 19.]
[Illustration: FIG. 20.]
Let it be supposed that an originally consequent river system has lived
into advanced maturity on a surface whose structure is, like that of
Pennsylvania, composed of closely adjacent anticlinal and synclinal
folds with rising and falling axes, and that a series of particularly
resistant beds composes the upper members of the folded mass. The
master stream, A, fig. 19, at maturity still resides where the original
folds were lowest, but the side streams have departed more less from
the axes of the synclinals that they first followed, in accordance with
the principles of adjustment presented above. The relief of the surface
is moderate, except around the synclinal troughs, where the rising
margins of the hard beds still appear as ridges of more or less
prominence. The minute hachures in figure 19 are drawn on the outcrop
side of these ridges. Now suppose a general elevation of the region,
lifting the synclinal troughs of the hard beds up to baselevel or even
somewhat above it. The deepening of the revived master-stream will be
greatly retarded by reason of its having to cross so many outcrops of
the hard beds, and thus excellent opportunity will be given for
readjustment by the growth of some diverting stream, B, whose beginning
on adjacent softer rocks was already made in the previous cycle. This
will capture the main river at some up-stream point, and draw it nearly
all away from its hard path across the synclinal troughs to an easier
path across the lowlands that had been opened on the underlying softer
beds, leaving only a small beheaded remnant in the lower course. The
final re-arrangement may be indicated in fig. 20. It should be noted
that every capture of branches of the initial main stream made by the
diverting stream adds to its ability for further encroachments, for
with increase of volume the channel is deepened and a flatter slope is
assumed, and the whole process of pushing away the divides is thereby
accelerated. In general it may be said that the larger the stream and
the less its elevation above baselevel, the less likely is it to be
diverted, for with large volume and small elevation it will early cut
down its channel so close to baselevel that no other stream can offer
it a better course to the sea; it may also be said that, as a rule, of
two equal streams, the headwaters of the one having a longer or a
harder course will be diverted by a branch of the stream on the shorter
or easier course. Every case must therefore be examined for itself
before the kind of re-arrangement that may be expected or that may have
already taken place can be discovered.
21. _Antecedent and superimposed rivers_.--It not infrequently
happens that the surface, on which a drainage system is more or
less fully developed, suffers deformation by tilting, folding or
faulting. Then, in accordance with the rate of disturbance, and
dependent on the size and slope of the streams and the resistance of
the rocks, the streams will be more or less re-arranged, some of the
larger ones persisting in their courses and cutting their channels down
almost as fast as the mass below them is raised and offered to their
action. It is manifest that streams of large volume and considerable
slope are the ones most likely to persevere in this way, while small
streams and large ones of moderate slope may be turned from their
former courses to new courses consequent on the new constructional form
of the land. Hence, after a disturbance, we may expect to find the
smaller streams of the former cycle pretty completely destroyed, while
some of the larger ones may still persist; these would then be called
antecedent streams in accordance with the nomenclature introduced by
Powell.[19] A fuller acquaintance with the development of our rivers
will probably give us examples of river systems of all degrees of
extinction or persistence at times of disturbance.
[Footnote 19: Exploration of the Colorada River of the West, 1875, 153,
163-166.]
Since Powell introduced the idea of antecedent valleys and Tietze,
Medlicott and others showed the validity of the explanation in other
regions than the one for which it was first proposed, it has found much
acceptance. Löwl's objection to it does not seem to me to be nearly so
well founded as his suggestion of an additional method of river
development by means of backward headwater erosion and subsequent
capture of other streams, as already described. And yet I cannot help
thinking that the explanation of transverse valleys as antecedent
courses savors of the Gordian method of explaining a difficult matter.
The case of the Green river, to which Powell first gave this
explanation, seems well supported; the examples given by Medlicott in
the Himalayas are as good: but still it does not seem advisable to
explain all transverse streams in this way, merely because they are
transverse. Perhaps one reason why the explanation has become so
popular is that it furnishes an escape from the old catastrophic idea
that fractures control the location of valleys, and is at the same time
fully accordant with the ideas of the uniformitarian school that have
become current in this half of our century. But when it is remembered
that most of the streams of a region are extinguished at the time of
mountain growth, that only a few of the larger ones can survive, and
that there are other ways in which transverse streams may
originate,[20] it is evident that the possibility of any given
transverse stream being antecedent must be regarded only as a
suggestion, until some independent evidence is introduced in its favor.
This may be difficult to find, but it certainly must be searched for;
if not then forthcoming, the best conclusion may be to leave the case
open until the evidence appears. Certainly, if we find a river course
that is accordant in its location with the complicated results of other
methods of origin, then the burden of proof may be said to lie with
those who would maintain that an antecedent origin would locate the
river in so specialized a manner. Even if a river persist for a time in
an antecedent course, this may not prevent its being afterwards
affected by the various adjustments and revivals that have been
explained above: rivers so distinctly antecedent as the Green and the
Sutlej may hereafter be more or less affected by processes of
adjustment, which they are not yet old enough to experience. Hence in
mountains as old as the Appalachians the courses of the present rivers
need not coincide with the location of the pre-Permian rivers, even if
the latter persisted in their courses through the growth of the Permian
folding; subsequent elevations and adjustments to hard beds, at first
buried and unseen, may have greatly displaced them, in accordance with
Löwl's principle.
[Footnote 20: Hilber, Pet. Mitth., xxxv, 1889, 13.]
When the deeper channelling of a stream discovers an unconformable
subjacent terrane, the streams persist at least for a time in the
courses that were determined in the overlying mass; they are then
called superimposed (Powell), inherited (Shaler), or epigenetic
(Richthofen). Such streams are particularly liable to readjustment by
transfer of channels from courses that lead them over hard beds to
others on which the hard beds are avoided; for the first choice of
channels, when the unconformable cover was still present, was made
without any knowledge of the buried rock structure or of the
difficulties in which the streams would be involved when they
encountered it. The examples of falls produced when streams terrace
their flood-plains and run on buried spurs has already been referred to
as superimposed; and the rivers of Minnesota now disclosing half-buried
ledges here and there may be instanced as illustrating the transition
stage between simple consequent courses, determined by the form of the
drift sheet on which their flow began, and the fully inconsequent
courses that will be developed there in the future.
22. _Simple, compound, composite and complex rivers_.--We have thus far
considered an ideal river. It now seems advisable to introduce a few
terms with which to indicate concisely certain well marked
peculiarities in the history of actual rivers.
An original river has already been defined as one which first takes
possession of a land area, or which replaces a completely extinguished
river on a surface of rapid deformation.
A river may be simple, if its drainage area is of practically one kind
of structure and of one age; like the rivers of southern New Jersey.
Such rivers are generally small. It may be composite, when drainage
areas of different structure are included in the basin of a single
stream. This is the usual case.
A compound river is one which is of different ages in its different
parts; as certain rivers of North Carolina, which have old headwaters
rising in the mountains, and young lower courses traversing the coastal
plain.
A river is complex when it has entered a second or later cycle of
development; the headwaters of a compound river are therefore complex,
while the lower course may be simple, in its first cycle. The degree of
complexity measures the number of cycles that the river has entered.
When the study of rivers is thus attempted, its necessary complications
may at first seem so great as to render it of no value; but in answer
to this I believe that it may be fairly urged that, although
complicated, the results are true to nature, and if so, we can have no
ground of complaint against them. Moreover, while it is desirable to
reduce the study of the development of rivers to its simplest form, in
order to make it available for instruction and investigation, it must
be remembered that this cannot be done by neglecting to investigate the
whole truth in the hope of avoiding too great complexity, but that
simplicity can be reached safely only through fullness of knowledge, if
at all.
It is with these points in mind that I have attempted to decipher the
history of the rivers of Pennsylvania. We find in the Susquehanna,
which drains a great area in the central part of the state, an example
of a river which is at once composite, compound and highly complex. It
drains districts of divers structure; it traverses districts of
different ages; and it is at present in its fourth or fifth degree of
complexity, its fourth or fifth cycle of development at least. In
unravelling its history and searching out the earlier courses of
streams which may have long since been abandoned in the processes of
mature adjustment, it will be seen that the size of the present streams
is not always a measure of their previous importance, and to this we
may ascribe the difficulty that attends the attempt to decipher a
river's history from general maps of its stream lines. Nothing but a
detailed examination of geological structure and history suffices to
detect facts and conditions that are essential to the understanding of
the result.
If the postulates that I shall use seem unsound and the arguments seem
overdrawn, error may at least be avoided by not holding fast to the
conclusions that are presented, for they are presented only
tentatively. I do not feel by any means absolutely persuaded of the
correctness of the results, but at the same time deem them worth giving
out for discussion. The whole investigation was undertaken as an
experiment to see where it might lead, and with the hope that it might
lead at least to a serious study of our river problems.
PART FOURTH. _The development of the rivers of Pennsylvania_.
23. _Means of distinguishing between antecedent and adjusted consequent
rivers_.--The outline of the geological history of Pennsylvania given
above affords means of dividing the long progress of the development of
our rivers into the several cycles which make up their complete life.
We must go far back into the past and imagine ancient streams flowing
down from the Archean land towards the paleozoic sea; gaining length by
addition to their lower portions as the land grew with the building on
of successive mountain ranges; for example, if there were a
Cambro-Silurian deformation, a continuation of the Green Mountains into
Pennsylvania, we suppose that the pre-existent streams must in some
manner have found their way westward to the new coastline; and from the
date of this mountain growth, it is apparent that any streams then born
must have advanced far in their history before the greater Appalachian
disturbance began. At the beginning of the latter, as of the former,
there must have been streams running from the land into the sea, and at
times of temporary elevation of the broad sand-flats of the coal
measures, such streams must have had considerable additions to their
lower length; rising in long-growing Archean highlands or mountains,
snow-capped and drained by glaciers for all we can say to the contrary,
descending across the Green Mountain belt, by that time worn to
moderate relief in the far advanced stage of its topographic
development, and finally flowing across the coal-measure lowlands of
recent appearance. It was across the lower courses of such rivers that
the Appalachian folds were formed, and the first step in our problem
consists in deciding if possible whether the streams held their courses
after the antecedent fashion, or whether they were thrown into new
courses by the growing folds, so that a new drainage system would be
formed. Possibly both conditions prevailed; the larger streams holding
their courses little disturbed, and the smaller ones disappearing, to
be replaced by others as the slopes of the growing surface should
demand. It is not easy to make choice in this matter. To decide that
the larger streams persisted and are still to be seen in the greater
rivers of to-day, only reversed in direction of flow, is certainly a
simple method of treating the problem, but unless some independent
reasons are found for this choice, it savors of assumption. Moreover,
it is difficult to believe that any streams, even if antecedent and
more or less persistent for a time during the mountain growth, could
preserve till now their pre-Appalachian courses through all the varying
conditions presented by the alternations of hard and soft rocks through
which they have had to cut, and at all the different altitudes above
baselevel in which they have stood. A better means of deciding the
question will be to admit provisionally the occurrence of a completely
original system of consequent drainage, located in perfect accord with
the slopes of the growing mountains; to study out the changes of
stream-courses that would result from later disturbances and from the
mutual adjustments of the several members of such a system in the
different cycles of its history; and finally to compare the courses
thus deduced with those now seen. If there be no accord, either the
method is wrong or the streams are not consequent but of some other
origin, such as antecedent; if the accord between deduction and fact be
well marked, varying only where no definite location can be given to
the deduced streams, but agreeing where they can be located more
precisely, then it seems to me that the best conclusion is distinctly
in favor of the correctness of the deductions. For it is not likely,
even if it be possible, that antecedent streams should have
accidentally taken, before the mountains were formed, just such
locations as would have resulted from the subsequent growth of the
mountains and from the complex changes in the initial river courses due
to later adjustments. I shall therefore follow the deductive method
thus indicated and attempt to trace out the history of a completely
original, consequent system of drainage accordant with the growth of
the central mountain district.
In doing this, it is first necessary to restore the constructional
topography of the region; that is, the form that the surface would have
had if no erosion had accompanied its deformation. This involves
certain postulates which must be clearly conceived if any measure of
confidence is to be gained in the results based upon them.
24. _Postulates of the argument_.--In the first place, I assume an
essential constancy in the thickness of the paleozoic sediments over
the entire area in question. This is warranted here because the known
variations of thickness are relatively of a second order, and will not
affect the distribution of high and low ground as produced by the
intense Permian folding. The reasons for maintaining that the whole
series had a considerable extension southeast of the present margin of
the Medina sandstone have already been presented.
In the second place, I shall assume that the dips and folds of the beds
now exposed at the surface of the ground may be projected upwards into
the air in order to restore the form of the eroded beds. This is
certainly inadmissible in detail, for it cannot be assumed that the
folded slates and limestones of the Nittany valley, for instance, give
any close indication of the form that the coal measures would have
taken, had they extended over this district, unworn. But in a general
way, the Nittany massif was a complex arch in the coal measures as well
as in the Cambrian beds; for our purpose and in view of the moderate
relief of the existing topography, it suffices to say that wherever the
lower rocks are now revealed in anticlinal structure, there was a great
upfolding and elevation of the original surface; and wherever the
higher rocks are still preserved, there was a relatively small
elevation.
In the third place, I assume that by reconstructing from the completed
folds the form which the country would have had if unworn, we gain a
sufficiently definite picture of the form through which it actually
passed at the time of initial and progressive folding. The difference
between the form of the folds completely restored and the form that the
surface actually reached is rather one of degree than of kind; the two
must correspond in the general distribution of high and low ground and
this is the chief consideration in our problem. When we remember how
accurately water finds its level, it will be clearer that what is
needed in the discussion is the location of the regions that were
relatively raised and lowered, as we shall then have marked out the
general course of the consequent water ways and the trend of the
intervening constructional ridges.
Accepting these postulates, it may be said in brief that the outlines
of the formations as at present exposed are in effect so many contour
lines of the old constructional surface, on which the Permian rivers
took their consequent courses. Where the Trenton limestone is now seen,
the greatest amount of overlying strata must have been removed; hence
the outline of the Trenton formation is our highest contour line. Where
the Helderberg limestone appears, there has been a less amount of
material removed; hence the Helderberg outcrop is a contour of less
elevation. Where the coal beds still are preserved, there has been
least wasting, and these beds therefore mark the lowest contour of the
early surface. It is manifest that this method assumes that the present
outcrops are on a level surface; this is not true, for the ridges
through the State rise a thousand feet more or less over the
intervening valley lowlands, and yet the existing relief does not count
for much in discussing the enormous relief of the Permian surface that
must have been measured in tens of thousands of feet at the time of its
greatest strength.
[Illustration: FIG. 21. Constructional Permian topography of
Pennsylvania.]
25. _Constructional Permian topography and consequent drainage_.--A
rough restoration of the early constructional topography is given in
fig. 21 for the central part of the State, the closest shading being
the area of the Trenton limestone, indicating the highest ground, or
better, the places of greatest elevation, while the Carboniferous area
is unshaded, indicating the early lowlands. The prevalence of northeast
and southwest trends was then even more pronounced than now. Several of
the stronger elements of form deserve names, for convenient reference.
Thus we have the great Kittatinny or Cumberland highland, C, C, on the
southeast, backed by the older mountains of Cambrian and Archean rocks,
falling by the Kittatinny slope to the synclinal lowland troughs of the
central district. In this lower ground lay the synclinal troughs of the
eastern coal regions, and the more local Broad Top basin, BT, on the
southwest, then better than now deserving the name of basins. Beyond
the corrugated area that connected the coal basins rose the great
Nittany highland, N, and its southwest extension in the Bedford range,
with the less conspicuous Kishicoquilas highland, K, in the foreground.
Beyond all stretched the great Alleghany lowland plains. The names thus
suggested are compounded of the local names of to-day and the
morphological names of Permian time.
What would be the drainage of such a country? Deductively we are led to
believe that it consisted of numerous streams as marked in full lines
on the figure, following synclinal axes until some master streams led
them across the intervening anticlinal ridges at the lowest points of
their crests and away into the open country to the northwest. All the
enclosed basins would hold lakes, overflowing at the lowest part of the
rim. The general discharge of the whole system would be to the
northwest. Here again we must resort to special names for the easy
indication of these well-marked features of the ancient and now
apparently lost drainage system. The master stream of the region is the
great Anthracite river, carrying the overflow of the Anthracite lakes
off to the northwest and there perhaps turning along one of the faintly
marked synclines of the plateau and joining the original Ohio, which
was thus confirmed in its previous location across the Carboniferous
marshes. The synclinal streams that entered the Anthracite lakes from
the southwest may be named, beginning on the south, the Swatara, S,
fig. 21, the Wiconisco, Wo, the Tuscarora-Mahanoy, M, the
Juniata-Catawissa, C, and the Wyoming, Wy. One of these, probably the
fourth, led the overflow from the Broad Top lake into the Catawissa
lake on the middle Anthracite river. The Nittany highland formed a
strong divide between the central and northwestern rivers, and on its
outer slope there must have been streams descending to the Alleghany
lowlands; and some of these may be regarded as the lower courses of
Carboniferous rivers, that once rose in the Archean mountains, now
beheaded by the growth of mountain ranges across their middle.
26. _The Jura mountains homologous with the Permian
Alleghanies_.--However willing one may be to grant the former existence
of such a drainage system as the above, an example of a similar one
still in existence would be acceptable as a witness to the
possibilities of the past. Therefore we turn for a moment to the Jura
mountains, always compared to the Appalachians on account of the
regular series of folds by which the two are characterized. But while
the initial topography is long lost in our old mountains, it is still
clearly perceptible in the young Jura, where the anticlines are still
ridges and the longitudinal streams still follow the synclinal troughs;
while the transverse streams cross from one synclinal valley to another
at points where the intervening anticlinal arches are lowest.[21] We
could hardly ask for better illustration of the deductive drainage
system of our early Appalachians than is here presented.
[Footnote 21: This is beautifully illustrated in the recent monograph
by La Noë and Margerie on "Les Formes du Terrain."]
27. _Development and adjustment of the Permian drainage_.--The problem
is now before us. Can the normal sequence of changes in the regular
course of river development, aided by the post-Permian deformations and
elevations, evolve the existing rivers out of the ancient ones?
In order to note the degree of comparison that exists between the two,
several of the larger rivers of to-day are dotted on the figure. The
points of agreement are indeed few and small. Perhaps the most
important ones are that the Broad Top region is drained by a stream,
the Juniata, which for a short distance follows near the course
predicted for it; and that the Nittany district, then a highland, is
still a well-marked divide although now a lowland. But there is no
Anthracite river, and the region of the ancient coal-basin lakes is now
avoided by large streams; conversely, a great river--the
Susquehanna--appears where no consequent river ran in Permian time, and
the early synclinal streams frequently turn from the structural troughs
to valleys located on the structural arches.
28. _Lateral water-gaps near the apex of synclinal ridges_.--One of the
most frequent discrepancies between the hypothetical and actual streams
is that the latter never follow the axis of a descending syncline along
its whole length, as the original streams must have done, but depart
for a time from the axis and then return to it, notching the ridge
formed on any hard bed at the side instead of at the apex of its curve
across the axis of the syncline. There is not a single case in the
state of a stream cutting a gap at the apex of such a synclinal curve,
but there are perhaps hundreds of cases where the streams notch the
curve to one side of the apex. This, however, is precisely the
arrangement attained by spontaneous adjustment from an initial axial
course, as indicated in figure 13. The gaps may be located on small
transverse faults, but as a rule they seem to have no such guidance. It
is true that most of our streams now run out of and not into the
synclinal basins, but a reason for this will be found later; for the
present we look only at the location of the streams, not at their
direction of flow. As far as this illustration goes, it gives evidence
that the smaller streams at least possess certain peculiarities that
could not be derived from persistence in a previous accidental
location, but which would be necessarily derived from a process of
adjustment following the original establishment of strictly consequent
streams. Hence the hypothesis that these smaller streams were long ago
consequent on the Permian folding receives confirmation; but this says
nothing as to the origin of the larger rivers, which might at the same
time be antecedent.
29. _Departure of the Juniata from the Juniata-Catawissa syncline_.--It
may be next noted that the drainage of the Broad Top region does not
follow a single syncline to the Anthracite region, as it should have in
the initial stage of the consequent Permian drainage, but soon turns
aside from the syncline in which it starts and runs across country to
the Susquehanna. It is true that in its upper course the Juniata
departs from the Broad Top region by one of the two synclines that were
indicated as the probable line of discharge of the ancient Broad Top
lake in our restoration of the constructional topography of the State;
there does not appear to be any significant difference between the
summit altitudes of the Tuscarora-Mahanoy and the Juniata-Catawissa
synclinal axes and hence the choice must have been made for reasons
that cannot be detected; or it may be that the syncline lying more to
the northwest was raised last, and for this reason was taken as the
line of overflow. The beginning of the river is therefore not
discordant with the hypothesis of consequent drainage, but the
southward departure from the Catawissa syncline at Lewistown remains to
be explained. It seems to me that some reason for the departure may be
found by likening it to the case already given in figs. 16-18. The
several synclines with which the Juniata is concerned have precisely
the relative attitudes that are there discussed. The Juniata-Catawissa
syncline has parallel sides for many miles about its middle, and hence
must have long maintained the initial Juniata well above baselevel over
all this distance; the progress of cutting down a channel through all
the hard Carboniferous sandstones for so great a distance along the
axis must have been exceedingly slow. But the synclines next south, the
Tuscarora-Mahanoy and the Wiconisco, plunge to the northeast more
rapidly, as the rapid divergence of their margins demonstrates, and
must for this reason have carried the hard sandstones below baselevel
in a shorter distance and on a steeper slope than in the Catawissa
syncline. The further southwestward extension of the Pocono sandstone
ridges in the southern than in the northern syncline gives further
illustration of this peculiarity of form. Lateral capture of the
Juniata by a branch of the initial Tuscarora, and of the latter by a
branch of the Wiconisco therefore seems possible, and the accordance of
the facts with so highly specialized an arrangement is certainly again
indicative of the correctness of the hypothesis of consequent drainage,
and this time in a larger stream than before. At first sight, it
appears that an easier lateral capture might have been made by some of
the streams flowing from the outer slope of the Nittany highland; but
this becomes improbable when it is perceived that the heavy Medina
sandstone would here have to be worn through as well as the repeated
arches of the Carboniferous beds in the many high folds of the Seven
Mountains. Again, as far as present appearances go, we can give no
sufficient reason to explain why possession of the headwaters of the
Juniata was not gained by some subsequent stream of its own, such as G,
fig. 18, instead of by a side-stream of the river in the neighboring
syncline; but it may be admitted, on the other hand, that as far as we
can estimate the chances for conquest, there was nothing distinctly in
favor of one or the other of the side-streams concerned; and as long as
the problem is solved indifferently in favor of one or the other, we
may accept the lead of the facts and say that some control not now
apparent determined that the diversion should be, as drawn, through D
and not through G. The detailed location of the Juniata in its middle
course below Lewistown will be considered in a later section.
[Illustration: FIG. 22.]
[Illustration: FIG. 23.]
[Illustration: FIG. 24.]
30. _Avoidance of the Broad Top basin by the Juniata
headwaters_.--Another highly characteristic change that the Juniata has
suffered is revealed by examining the adjustments that would have taken
place in the general topography of the Broad Top district during the
Perm-Triassic cycle of erosion. When the basin, BT, fig. 22, was first
outlined, centripetal streams descended its slopes from all sides and
their waters accumulated as a lake in the center, overflowing to the
east into the subordinate basin, A, in the Juniata syncline along side
of the larger basin, and thence escaping northeast. In due time, the
breaching of the slopes opened the softer Devonian rocks beneath and
peripheral lowlands were opened on them. The process by which the
Juniata departed from its original axial location, J, fig. 22, to a
parallel course on the southeastern side of the syncline, J, fig. 23,
has been described (fig. 18). The subsequent changes are manifest. Some
lateral branch of the Juniata, like N, fig. 23, would work its way
around the northern end of the Broad Top canoe on the soft underlying
rocks and capture the axial stream, C, that came from the depression
between Nittany and Kishicoquillas highlands; thus reënforced, capture
would be made of a radial stream from the west, Tn, the existing Tyrone
branch of the Juniata; in a later stage the other streams of the
western side of the basin would be acquired, their divertor
constituting the Little Juniata of to-day; and the end would be when
the original Juniata, A, fig. 22, that once issued from the subordinate
synclinal as a large stream, had lost all its western tributaries, and
was but a shrunken beheaded remnant of a river, now seen in Aughwick
creek, A, fig. 24. In the meantime, the former lake basin was fast
becoming a synclinal mountain of diminishing perimeter. The only really
mysterious courses of the present streams are where the Little Juniata
runs in and out of the western border of the Broad Top synclinal, and
where the Frankstown (FT) branch of the Juniata maintains its
independent gap across Tussey's mountain (Medina), although diverted to
the Tyrone or main Juniata (Tn) by Warrior's ridge (Oriskany) just
below. At the time of the early predatory growth of the initial
divertor, N, its course lay by the very conditions of its growth on
only the weakest rocks; but after this little stream had grown to a
good-sized river, further rising of the land, probably in the time of
the Jurassic elevation, allowed the river to sink its channel to a
greater depth, and in doing so, it encountered the hard Medina
anticline of Jack's mountain; here it has since persisted, because, as
we may suppose, there has been no stream able to divert the course of
so large a river from its crossing of a single hard anticlinal.
The doubt that one must feel as to the possibility of the processes
just outlined arises, if I may gauge it by my own feeling, rather from
incredulity than from direct objections. It seems incredible that the
waste of the valley slopes should allow the backward growth of N at
such a rate as to enable it to capture the heads of C, Tn, F, and so
on, before they had cut their beds down close enough to the baselevel
of the time to be safe from capture. But it is difficult to urge
explicit objections against the process or to show its quantitative
insufficiency. It must be remembered that when these adjustments were
going on, the region was one of great altitude, its rocks then had the
same strong contrasts of strength and weakness that are so apparent in
the present relief of the surface and the streams concerned were of
moderate size; less than now, for at the time, the Tyrone, Frankstown
and Bedford head branches of the Juniata had not acquired drainage west
of the great Nittany-Bedford anticlinal axis, but were supplied only by
the rainfall on its eastern slope (see section 39)--and all these
conditions conspired to favor the adjustment. Finally, while apparently
extraordinary and difficult of demonstration, the explanation if
applicable at all certainly gives rational correlation to a number of
peculiar and special stream courses in the upper Juniata district that
are meaningless under any other theory that has come to my notice. It
is chiefly for this reason that I am inclined to accept the
explanation.
31. _Reversal of larger rivers to southeast courses_.--Our large rivers
at present flow to the southeast, not to the northwest. It is difficult
to find any precise date for this reversal of flow from the initial
hypothetical direction, but it may be suggested that it occurred about
the time of the Triassic depression of the Newark belt. We have been
persuaded that much time elapsed between the Permian folding and the
Newark deposition, even under the most liberal allowance for
pre-Permian erosion in the Newark belt; hence when the depression
began, the rivers must have had but moderate northwestward declivity.
The depression and submergence of the broad Newark belt may at this
time have broken the continuity of the streams that once flowed across
it. The headwater streams from the ancient Archean country maintained
their courses to the depression; the lower portions of the rivers may
also have gone on as before; but the middle courses were perhaps turned
from the central part of the state back of the Newark belt. No change
of attitude gives so fitting a cause of the southeastward flow of our
rivers as this. The only test that I have been able to devise for the
suggestion is one that is derived from the relation that exists between
the location of the Newark belt along the Atlantic slope and the course
of the neighboring transverse rivers. In Pennsylvania, where the belt
reaches somewhat beyond the northwestern margin of the crystalline
rocks in South mountain, the streams are reversed, as above stated; but
in the Carolinas where the Newark belt lies far to the east of the
boundary between the Cambrian and crystalline rocks, the Tennessee
streams persevere in what we suppose to have been their original
direction of flow. This may be interpreted as meaning that in the
latter region, the Newark depression was not felt distinctly enough, if
at all, within the Alleghany belt to reverse the flow of the streams;
while in the former region, it was nearer to these streams and
determined a change in their courses. The original Anthracite river ran
to the northwest, but its middle course was afterwards turned to the
southeast.
I am free to allow that this has the appearance of heaping hypothesis
on hypothesis; but in no other way does the analysis of the history of
our streams seem possible, and the success of the experiment can be
judged only after making it. At the same time, I am constrained to
admit that this is to my own view the least satisfactory of the
suggestions here presented. It may be correct, but there seems to be no
sufficient exclusion of other possibilities. For example, it must not
be overlooked that, if the Anthracite river ran southeast during Newark
deposition, the formation of the Newark northwestward monocline by the
Jurassic tilting would have had a tendency to turn the river back again
to its northwest flow. But as the drainage of the region is still
southeastward, I am tempted to think that the Jurassic tilting was not
here strong enough to reverse the flow of so strong and mature a river
as the Anthracite had by that time come to be; and that the elevation
that accompanied the tilting was not so powerful in reversing the river
to a northwest course as the previous depression of the Newark basin
had been in turning it to the southeast. If the Anthracite did continue
to flow to the southeast, it may be added that the down-cutting of its
upper branches was greatly retarded by the decrease of slope in its
lower course when the monocline was formed.
The only other method of reversing the original northwestward flow of
the streams that I have imagined is by capture of their headwaters by
Atlantic rivers. This seems to me less effective than the method just
considered; but they are not mutually exclusive and the actual result
may be the sum of the two processes. The outline of the idea is as
follows. The long continued supply of sedimentary material from the
Archean land on the southeast implies that it was as continually
elevated. But there came a time when there is no record of further
supply of material, and when we may therefore suppose the elevation was
no longer maintained. From that time onward, the Archean range must
have dwindled away, what with the encroachment of the Atlantic on its
eastern shore and the general action of denuding forces on its surface.
The Newark depression was an effective aid to the same end, as has been
stated above, and for a moderate distance westward of the depressed
belt, the former direction of the streams must certainly have been
reversed; but the question remains whether this reversal extended as
far as the Wyoming basin, and whether the subsequent formation of the
Newark monocline did not undo the effect of the Newark depression. It
is manifest that as far as our limited knowledge goes, it is impossible
to estimate these matters quantitatively, and hence the importance of
looking for additional processes that may supplement the effect of the
Newark depression and counteract the effect of the Newark uplift in
changing the course of the rivers. Let it be supposed for the moment
that at the end of the Jurassic uplift by which the Newark monocline
was formed, the divide between the Ohio and the Atlantic drainage lay
about the middle of the Newark belt. There was a long gentle descent
westward from this watershed and a shorter and hence steeper descent
eastward. Under such conditions, the divide must have been pushed
westward, and as long as the rocks were so exposed as to open areas of
weak sediments on which capture by the Atlantic streams could go on
with relative rapidity, the westward migration of the divide would be
important. For this reason, it might be carried from the Newark belt as
far as the present Alleghany front, beyond which further pushing would
be slow, on account of the broad stretch of country there covered by
hard horizontal beds.
The end of this is that, under any of the circumstances here detailed,
there would be early in the Jurassic-Cretaceous cycle a distinct
tendency to a westward migration of the Atlantic-Ohio divide; it is the
consequences of this that have now to be examined.
32. _Capture of the Anthracite headwaters by the growing
Susquehanna_.--Throughout the Perm-Triassic period of denudation, a
great work was done in wearing down the original Alleghanies.
Anticlines of hard sandstone were breached, and broad lowlands were
opened on the softer rocks beneath. Little semblance of the early
constructional topography remained when the period of Newark depression
was brought to a close; and all the while the headwater streams of the
region were gnawing at the divides, seeking to develop the most perfect
arrangement of waterways. Several adjustments have taken place, and the
larger streams have been reversed in the direction of their flow; but a
more serious problem is found in the disappearance of the original
master stream, the great Anthracite river, which must have at first led
away the water from all the lateral synclinal streams. Being a large
river, it could not have been easily diverted from its course, unless
it was greatly retarded in cutting down its channel by the presence of
many beds of hard rocks on its way. The following considerations may
perhaps throw some light on this obscure point.
[Illustration: FIG. 25. General distribution of high and low land and
drainage in early Jurassic time.]
It may be assumed that the whole group of mountains formed by the
Permian deformation had been reduced to a moderate relief when the
Newark deposition was stopped by the Jurassic elevation. The harder
ribs of rock doubtless remained as ridges projecting above the
intervening lowlands, but the strength of relief that had been given by
the constructional forces had been lost. The general distribution of
residual elevations then remaining unsubdued is indicated in fig. 25,
in which the Crystalline, the Medina, and the two Carboniferous
sandstone ridges are denoted by appropriate symbols. In restoring this
phase of the surface form, when the country stood lower than now, I
have reduced the anticlines from their present outlines and increased
the synclines, the change of area being made greatest where the dips
are least, and hence most apparent at the ends of the plunging
anticlines and synclines. Some of the Medina anticlines of Perry and
Juniata counties are not indicated because they were not then
uncovered. The country between the residual ridges of Jurassic time was
chiefly Cambrian limestone and Siluro-Devonian shales and soft
sandstones. The moderate ridges developed on the Oriskany and Chemung
sandstones are not represented. The drainage of this stage retained the
original courses of the streams, except for the adjustments that have
been described, but the great Anthracite river is drawn as if it had
been controlled by the Newark depression and reversed in the direction
of its flow, so that its former upper course on the Cambrian rocks was
replaced by a superimposed Newark lower course. Fig. 25 therefore
represents the streams for the most part still following near their
synclinal axes, although departing from them where they have to enter a
synclinal cove-mountain ridge; the headwaters of the Juniata avoid the
mass of hard sandstones discovered in the bottom of old Broad Top lake,
and flow around them to the north, and then by a cross-country course
to the Wiconisco synclinal, as already described in detail. Several
streams come from the northeast, entering the Anthracite district after
the fashion generalized in fig. 13. Three of the many streams that were
developed on the great Kittatinny slope are located, with their
direction of flow reversed; these are marked Sq, L and D, and are
intended to represent the ancestors of the existing Susquehanna, Lehigh
and Delaware. We have now to examine the opportunities offered to these
small streams to increase their drainage areas.
The Jurassic elevation, by which the Newark deposition was stopped,
restored to activity all the streams that had in the previous cycle
sought and found a course close to baselevel. They now all set to work
again deepening their channels. But in this restoration of lost
activity with reference to a new baselevel, there came the best
possible chance for numerous re-arrangements of drainage areas by
mutual adjustment into which we must inquire.
I have already illustrated what seems to me to be the type of the
conditions involved at this time in figs. 19 and 20. The master stream,
A, traversing the synclines, corresponds to the reversed Anthracite
river; the lowlands at the top are those that have been opened out on
the Siluro-Devonian beds of the present Susquehanna middle course
between the Pocono and the Medina ridges. The small stream, B, that is
gaining drainage area in these lowlands, corresponds to the embryo of
the present Susquehanna, Sq, fig. 25, this having been itself once a
branch on the south side of the Swatara synclinal stream, fig. 21, from
which it was first turned by the change of slope accompanying the
Newark depression; but it is located a little farther west than the
actual Susquehanna, so as to avoid the two synclinal cove mountains of
Pocono sandstone that the Susquehanna now traverses, for reasons to be
stated below (section 35). This stream had to cross only one bed of
hard rock, the outer wall of Medina sandstone, between the broad inner
lowlands of the relatively weak Siluro-Devonian rocks and the great
valley lowlands on the still weaker Cambrian limestones. Step by step
it must have pushed its headwater divide northward, and from time to
time it would have thus captured a subsequent stream, that crossed the
lowlands eastward, and entered a Carboniferous syncline by one of the
lateral gaps already described. With every such capture, the power of
the growing stream to capture others was increased. Fig. 19 represents
a stage after the streams in the Swatara and Wiconisco synclines (the
latter then having gained the Juniata) had been turned aside on their
way to the Carboniferous basins. On the other hand, the Anthracite
river, rising somewhere on the plains north of the Wyoming syncline and
pursuing an irregular course from one coal basin to another, found an
extremely difficult task in cutting down its channel across the
numerous hard beds of the Carboniferous sandstones, so often repeated
in the rolling folds of the coal fields. It is also important to
remember that an aid to other conditions concerned in the diversion of
the upper Anthracite is found in the decrease of slope that its lower
course suffered in crossing the coal fields, if that area took any part
in the deformation that produced the Newark monocline--whichever theory
prove true in regard to the origin of the southeastward flow of the
rivers--for loss of slope in the middle course, where the river had to
cross many reefs of hard sandstone, would have been very effective in
lengthening the time allowed for the diversion of the headwaters.
The question is, therefore, whether the retardation of down-cutting
here experienced by the Anthracite was sufficient to allow the capture
of its headwaters by the Susquehanna. There can be little doubt as to
the correct quality of the process, but whether it was quantitatively
sufficient is another matter. In the absence of any means of testing
its sufficiency, may the result not be taken as the test? Is not the
correspondence between deduction and fact close enough to prove the
correctness of the deduction?
33. _Present outward drainage of the Anthracite basins_.--The Lehigh,
like the Susquehanna, made an attempt to capture the headwaters of
adjacent streams, but failed to acquire much territory from the
Anthracite because the Carboniferous sandstones spread out between the
two in a broad plateau of hard rocks, across which the divide made
little movement. The plateau area that its upper branches drain is, I
think, the conquest of a later cycle of growth. The Delaware had little
success, except as against certain eastern synclinal branches of the
Anthracite, for the same reason. The ancestor of the Swatara of to-day
made little progress in extending its headwaters because its point of
attack was against the repeated Carboniferous sandstones in the Swatara
synclinal. One early stream alone found a favorable opportunity for
conquest, and thus grew to be the master river--the Susquehanna of
to-day. The head of the Anthracite was carried away by this captor, and
its beheaded lower portion remains in our Schuylkill. The Anthracite
coal basins, formerly drained by the single master stream, have since
been apportioned to the surrounding rivers. As the Siluro-Devonian
lowlands were opened around the coal-basins, especially on the north
and west, the streams that formerly flowed into the basins were
gradually inverted and flowed out of them, as they still do. The extent
of the inversion seems to be in a general way proportionate to its
opportunity. The most considerable conquests were made in the upper
basins, where the Catawissa and Nescopec streams of to-day drain many
square miles of wide valleys opened on the Mauch Chunk red shale
between the Pocono and Pottsville sandstone ridges; the ancient middle
waters of the Anthracite here being inverted to the Susquehanna
tributaries, because the northern coal basins were degraded very slowly
after the upper Anthracite had been diverted. The Schuylkill as the
modern representative of the Anthracite retains only certain streams
south of a medial divide between Nescopec and Blue mountains. The only
considerable part of the old Anthracite river that still retains a
course along the axis of a synclinal trough seems to be that part which
follows the Wyoming basin; none of the many other coal basins are now
occupied by the large stream that originally followed them. The reason
for this is manifestly to be found in the great depth of the Wyoming
basin, whereby the axial portion of its hard sandstones are even now
below baselevel, and hence have never yet acted to throw the river from
its axial course. Indeed, during the early cycles of denudation, this
basin must have been changed from a deep lake to a lacustrine plain by
the accumulation in it of waste from the surrounding highlands, and for
a time the streams that entered it may have flowed in meandering
courses across the ancient alluvial surface; the lacustrine and
alluvial condition may have been temporarily revived at the time of the
Jurassic elevation. It is perhaps as an inheritance from a course thus
locally superimposed that we may come to regard the deflection of the
river at Nanticoke from the axis of the syncline to a narrow shale
valley on its northern side, before turning south again and leaving the
basin altogether. But like certain other suggestions, this can only be
regarded as an open hypothesis, to be tested by some better method of
river analysis than we now possess; like several of the other
explanations here offered, it is presented more as a possibility to be
discussed than as a conclusion to be accepted.
I believe that it was during the earlier part of the great
Jura-Cretaceous cycle of denudation that the Susquehanna thus became
the master stream of the central district of the state. For the rest of
the cycle, it was occupied in carrying off the waste and reducing the
surface to a well finished baselevel lowland that characterized the end
of Cretaceous time. From an active youth of conquest, the Susquehanna
advanced into an old age of established boundaries; and in later times,
its area of drainage does not seem to have been greatly altered from
that so long ago defined; except perhaps in the districts drained by
the West and North Branch headwaters.
34. _Homologies of the Susquehanna and Juniata_.--Looking at the change
from the Anthracite to the Susquehanna in a broad way, one may perceive
that it is an effect of the same order as the peripheral diversion of
the Broad Top drainage, illustrated in figures 22, 23 and 24; another
example of a similar change is seen in the lateral diversion of the
Juniata above Lewistown and its rectilinear continuation in Aughwick
creek, from their original axial location when they formed the initial
Broad Top outlet. They have departed from the axis of their syncline to
the softer beds on its southern side; FE of fig. 17 has been diverted
to FD of fig. 18.
All of these examples are truly only special cases of the one already
described in which the Juniata left its original syncline for others to
the south. The general case may be stated in a few words. A stream
flowing along a syncline of hard beds (Carboniferous sandstones)
develops side streams which breach the adjacent anticlines and open
lowlands in the underlying softer beds (Devonian and Silurian). On
these lowlands, the headwaters of side streams from other synclines are
encountered and a contest ensues as to possession of the drainage
territory. The divides are pushed away from those headwaters whose
lower course leads them over the fewest hard barriers; this conquest
goes on until the upper course of the initial main stream is diverted
to a new and easier path than the one it chose in its youth in
obedience to the first deformation of the region. Thus the Juniata now
avoids the center and once deepest part of the old Broad Top lake,
because in the general progress of erosion, lowlands on soft Devonian
beds were opened all around the edge of the great mass of sandstones
that held the lake; the original drainage across the lake, from its
western slopes to its outlet just south of the Jack's mountain
anticline, has now taken an easier path along the Devonian beds to the
west of the old lake basin, and is seen in the Little Juniata, flowing
along the outer side of Terrace mountain and rounding the northern
synclinal point where Terrace mountain joins Sideling hill. It then
crosses Jack's mountain at a point where the hard Medina sandstones of
the mountain were still buried at the time of the choice of this
channel. In the same way, the drainage of the subordinate basin,
through which the main lake discharged eastward, is now not along the
axis of the Juniata-Catawissa syncline, but on the softer beds along
one side of it; and along the southern side because the easier escape
that was provided for it lay on that side, namely, via the Tuscarora
and Wiconisco synclines, as already described. The much broader change
from the Anthracite to the Susquehanna was only another form of the
same process. Taking a transverse view of the whole system of central
folds, it is perceived that their axes descend into the Anthracite
district from the east and rise westward therefrom; it is as if the
whole region had received a slight transverse folding, and the
transverse axis of depression thus formed defined the initial course of
the first master stream. But this master stream deserted its original
course on the transverse axis of depression because a lateral course
across lowlands on softer beds was opened by its side streams; and in
the contest on these lowlands with an external stream, the Susquehanna,
the upper portion of the Anthracite was diverted from the hard rocks
that had appeared on the transverse axis. The distance of diversion
from the axial to the lateral course in this case was great because of
the gentle quality of the transverse folding; or, better said, because
of the gentle dips of the axes of the longitudinal folds. This
appearance of systematic re-arrangement in the several river courses
where none was expected is to my mind a strong argument in favor of the
originally consequent location of the rivers and their later mutual
adjustment. It may perhaps be conceived that antecedent streams might
imitate one another roughly in the attitude that they prophetically
chose with regard to folds subsequently formed, but no reason has been
suggested for the imitation being carried to so remarkable and definite
a degree as that here outlined.
35. _Superimposition of the Susquehanna on two synclinal
ridges_.--There is however one apparently venturesome postulate that
may have been already noted as such by the reader; unless it can be
reasonably accounted for and shown to be a natural result of the long
sequence of changes here considered, it will seriously militate against
the validity of the whole argument. The present course of the middle
Susquehanna leads it through the apical curves of two Pocono synclinal
ridges, which were disregarded in the statement given above. It was
then assumed that the embryonic Susquehanna gained possession of the
Siluro-Devonian lowland drainage by gnawing out a course to the west of
these synclinal points; for it is not to be thought of that any
conquest of the headwaters of the Anthracite river could have been made
by the Susquehanna if it had had to gnaw out the existing four
traverses of the Pocono sandstones before securing the drainage of the
lowlands above them. The backward progress of the Susquehanna could not
in that case have been nearly fast enough to reach the Anthracite
before the latter had sunk its channel to a safe depth. It is therefore
important to justify the assumption as to the more westerly location of
the embryonic Susquehanna; and afterwards, to explain how it should
have since then been transferred to its present course. A short cut
through all this round-about method is open to those who adopt in the
beginning the theory that the Susquehanna was an antecedent river; but
as I have said at the outset of this inquiry, it seems to me that such
a method is not freer from assumption, even though shorter than the one
here adopted; and it has the demerit of not considering all the curious
details that follow the examination of consequent and adjusted courses.
The sufficient reason for the assumption that the embryonic Susquehanna
lay farther west than the present one in the neighborhood of the Pocono
synclinals is simply that--in the absence of any antecedent stream--it
must have lain there. The whole explanation of the development of the
Siluro-Devonian lowlands between the Pocono and Medina ridges depends
simply on their being weathered out where the rocks are weak enough to
waste faster than the enclosing harder ridges through which the streams
escape. In this process, the streams exercise no control whatever over
the direction in which their headwaters shall grow; they leave this
entirely to the structure of the district that they drain. It thus
appears that, under the postulate as to the initial location of the
Susquehanna as one of the many streams descending the great slope of
the Kittatinny (Cumberland) highland into the Swatara syncline, its
course being reversed from northward to southward by the Newark
depression, we are required to suppose that its headwater (northward)
growth at the time of the Jurassic elevation must have been on the
Siluro-Devonian beds, so as to avoid the harder rocks on either side.
Many streams competed for the distinction of becoming the master, and
that one gained its ambition whose initial location gave it the best
subsequent opportunity. It remains then to consider the means by which
the course of the conquering Susquehanna may have been subsequently
changed from the lowlands on to the two Pocono synclines that it now
traverses. Some departure from its early location may have been due to
eastward planation in its advanced age, when it had large volume and
gentle slope and was therefore swinging and cutting laterally in its
lower course. This may have had a share in the result, but there is
another process that seems to me more effective.
In the latter part of the Jura-Cretaceous cycle, the whole country
hereabout suffered a moderate depression, by which the Atlantic
transgressed many miles inland from its former shoreline, across the
lowlands of erosion that had been developed on the litoral belt. Such a
depression must have had a distinct effect on the lower courses of the
larger rivers, which having already cut their channels down close to
baselevel and opened their valleys wide on the softer rocks, were then
"estuaried," or at least so far checked as to build wide flood-plains
over their lower stretches. Indeed, the flood-plains may have been
begun at an earlier date, and have been confirmed and extended in the
later time of depression. Is it possible that in the latest stage of
this process, the almost baselevelled remnants of Blue mountain and the
Pocono ridges could have been buried under the flood-plain in the
neighborhood of the river?
If this be admitted, it is then natural for the river to depart from
the line of its buried channel and cross the buried ridges on which it
might settle down as a superimposed river in the next cycle of
elevation. It is difficult to decide such general questions as these;
and it may be difficult for the reader to gain much confidence in the
efficacy of the processes suggested; but there are certain features in
the side streams of the Susquehanna that lend some color of probability
to the explanation as offered.
Admit, for the moment, that the aged Susquehanna, in the later part of
the Jura-Cretaceous cycle, did change its channel somewhat by cutting
to one side, or by planation, as it is called. Admit, also, that in the
natural progress of its growth it had built a broad flood plain over
the Siluro-Devonian lowlands, and that the depth of this deposit was
increased by the formation of an estuarine delta upon it when the
country sank at the time of the mid-Cretaceous transgression of the
sea. It is manifest that one of the consequences of all this might be
the peculiar course of the river that is to be explained, namely, its
superimposition on the two Pocono synclinal ridges in the next cycle of
its history, after the Tertiary elevation had given it opportunity to
re-discover them. It remains to inquire what other consequences should
follow from the same conditions, and from these to devise tests of the
hypothesis.
36. _Evidence of superimposition in the Susquehanna tributaries_.--One
of the peculiarities of flood-plained rivers is that the lateral
streams shift their points of union with the main stream farther and
farther down the valley, as Lombardini has shown in the case of the Po.
If the Susquehanna were heavily flood-plained at the close of the
Jura-Cretaceous cycle, some of its tributaries should manifest signs of
this kind of deflection from their structural courses along the strike
of the rocks. Side streams that once joined the main stream on the line
of some of the softer northeast-southwest beds, leaving the stronger
beds as faint hills on either side, must have forgotten such control
after it was baselevelled and buried; as the flood plain grew, they
properly took more and more distinctly downward deflected courses, and
these deflections should be maintained in subsequent cycles as
superimposed courses independent of structural guidance. Such I believe
to be the fact. The downstream deflection is so distinctly a
peculiarity of a number of tributaries that join the Susquehanna on the
west side (see figure 1) that it cannot be ascribed to accident, but
must be referred to some systematic cause. Examples of deflection are
found in Penn's creek, Middle creek and North Mahantango creek in
Snyder county; West Mahantango between the latter and Juniata county;
and in the Juniata and Little Juniata rivers of Perry county. On the
other side of the Susquehanna, the examples are not so distinct, but
the following may be mentioned: Delaware and Warrior runs,
Chillisquaque creek and Little Shamokin creek, all in Northumberland
county. It may be remarked that it does not seem impossible that the
reason for the more distinct deflection of the western streams may be
that the Susquehanna is at present east of its old course, and hence
towards the eastern margin of its flood plain, as, indeed its position
on the Pocono synclinals implies. A reason for the final location of
the superimposed river on the eastern side of the old flood plain may
perhaps be found in the eastward tilting that is known to have
accompanied the elevation of the Cretaceous lowland.
It follows from the foregoing that the present lower course of the
Susquehanna must also be of superimposed origin; for the flood plain of
the middle course must have extended down stream to its delta, and
there have become confluent with the sheet of Cretaceous sediments that
covered all the southeastern lowland, over which the sea had
transgressed. McGee has already pointed out indications of superimposed
stream courses in the southeastern part of the State;[22] but I am not
sure that he would regard them as of the date here referred to.
[Footnote 22: Amer. Journ. Science, xxxv, 1888, 121, 134.]
The theory of the location of the Susquehanna on the Pocono synclinal
ridges therefore stands as follows. The general position of the river
indicates that it has been located by some process of slow
self-adjusting development and that it is not a persistent antecedent
river; and yet there is no reason to think that it could have been
brought into its present special position by any process of shifting
divides. The processes that have been suggested to account for its
special location, as departing slightly from a location due to slow
adjustments following an ancient consequent origin, call for the
occurrence of certain additional peculiarities in the courses of its
tributary streams, entirely unforeseen and unnoticed until this point
in the inquiry is reached; and on looking at the map to see if they
occur, they are found with perfect distinctness. The hypothesis of
superimposition may therefore be regarded as having advanced beyond the
stage of mere suggestion and as having gained some degree of
confirmation from the correlations that it detects and explains. It
only remains to ask if these correlations might have originated in any
other way, and if the answer to this is in the negative, the case may
be looked upon as having a fair measure of evidence in its favor. The
remaining consideration may be taken up at once as the first point to
be examined in the Tertiary cycle of development.
37. _Events of the Tertiary cycle_.--The elevation given to the region
by which Cretaceous baselevelling was terminated, and which I have
called the early Tertiary elevation, offered opportunity for the
streams to deepen their channels once more. In doing so, certain
adjustments of moderate amount occurred, which will be soon examined.
As time went on, much denudation was effected, but no wide-spread
baselevelling was reached, for the Cretaceous crest lines of the hard
sandstone ridges still exist. The Tertiary cycle was an incomplete one.
At its close, lowlands had been opened only on the weaker rocks between
the hard beds. Is it not possible that the flood-plaining of the
Susquehanna and the down-stream deflection of its branches took place
in the closing stages of this cycle, instead of at the end of the
previous cycle? If so, the deflection might appear on the branches, but
the main river would not be transferred to the Pocono ridges. This
question may be safely answered in the negative; for the Tertiary
lowland is by no means well enough baselevelled to permit such an
event. The beds of intermediate resistance, the Oriskany and certain
Chemung sandstones, had not been worn down to baselevel at the close of
the Tertiary cycle; they had indeed lost much of the height that they
possessed at the close of the previous cycle, but they had not been
reduced as low as the softer beds on either side. They were only
reduced to ridges of moderate and unequal height over the general plain
of the Siluro-Devonian low country, without great strength of relief
but quite strong enough to call for obedience from the streams along
side of them. And yet near Selin's Grove, for example, in Snyder
county, Penn's and Middle creeks depart most distinctly from the strike
of the local rocks as they near the Susquehanna, and traverse certain
well-marked ridges on their way to the main river. Such aberrant
streams cannot be regarded as superimposed at the close of the
incomplete Tertiary cycle; they cannot be explained by any process of
spontaneous adjustment yet described, nor can they be regarded as
vastly ancient streams of antecedent courses; I am therefore much
tempted to consider them as of superimposed origin, inheriting their
present courses from the flood-plain cover of the Susquehanna in the
latest stage of the Jura-Cretaceous cycle. With this tentative
conclusion in mind as to the final events of Jura-Cretaceous time, we
may take up the more deliberate consideration of the work of the
Tertiary cycle.
The chief work of the Tertiary cycle was merely the opening of the
valley lowlands; little opportunity for river adjustment occurred
except on a small scale. The most evident cases of adjustment have
resulted in the change of water-gaps into wind-gaps, of which several
examples can be given, the one best known being the Delaware wind-gap
between the Lehigh and Delaware water-gaps in Blue mountain. The
wind-gap marks the unfinished notch of some stream that once crossed
the ridge here and whose headwaters have since then been diverted,
probably to the Lehigh. The difficulty in the case is not at all how
the stream that once flowed here was diverted, but how a stream that
could be diverted in the Tertiary cycle could have escaped diversion at
some earlier date. The relative rarity of wind-gaps indicates that
nearly all of the initial lateral streams, which may have crossed the
ridges at an early epoch in the history of the rivers, have been
beheaded in some cycle earlier than the Tertiary and their gaps
thereafter obliterated. Why the Delaware wind-gap stream should have
endured into a later cycle does not at present appear. Other wind-gaps
of apparently similar origin may be found in Blue mountain west of the
Schuylkill and east of the Susquehanna. It is noteworthy that if any
small streams still persevere in their gaps across a hard ridge, they
are not very close to any large river-gap; hence it is only at the very
headwaters of Conedogwinet creek, in the northern part of Franklin
county, that any water is still drawn from the back of Blue mountain.
Again, these small stream gaps do not lie between large river-gaps and
wind-gaps, but wind-gaps lie between the gaps of large rivers and those
of small streams that are not yet diverted. Excellent illustration of
this is found on the "Piedmont sheet" of the contoured maps issued by
the United States Geological Survey. The sheet covers part of Maryland
and West Virginia, near where the North Branch of the Potomac comes out
of the plateau and crosses New Creek mountain. Eleven miles south of
the Potomac gap there is a deep wind-gap; but further on, at twenty,
twenty-five and twenty-nine miles from the river-gap are three fine
water-gaps occupied by small streams. This example merely shows how
many important points in the history of our rivers will be made clear
when the country is properly portrayed on contoured maps.
A few lines may be given to the general absence of gaps in Blue
Mountain in Pennsylvania. When the initial consequent drainage was
established, many streams must have been located on the northward slope
of the great Cumberland highland, C, C, fig. 21; they must have gullied
the slope to great depths and carried away great volumes of the weak
Cambrian beds that lay deep within the hard outer casings of the mass.
Minor adjustments served to diminish the number of these streams, but
the more effective cause of their present rarity lay in the natural
selection of certain of them to become large streams; the smaller ones
were generally beheaded by these. The only examples of streams that
still cross this ridge with their initial Permian direction of flow to
the northwest are found in two southern branches of Tuscarora creek at
the southern point of Juniata county; and these survive because of
their obscure location among the many Medina ridges of that district,
where they were not easily accessible to capture by other streams.
38. _Tertiary adjustment of the Juniata on the Medina anticlines_.--The
lower course of the Juniata presents several examples of adjustment
referable to the last part of the Jura-Cretaceous cycle and to the
Tertiary cycle. The explanation offered for the escape of this river
from its initial syncline did not show any reason for its peculiar
position with respect to the several Medina anticlines that it now
borders, because at the time when it was led across country to the
Wiconisco syncline, the hard Medina beds of these anticlines were not
discovered. It is therefore hardly to be thought that the location of
the Juniata in the Narrows below Lewistown between Blue Ridge and Shade
mountain and its avoidance of Tuscarora mountain could have been
defined at that early date. But all these Medina anticlines rise more
or less above the Cretaceous baselevel, and must have had some effect
on the position taken by the river about the middle of that cycle when
its channel sank upon them. Blue Ridge and Black Log anticlines rise
highest. The first location of the cross-country stream that led the
early Juniata away from its initial syncline probably traversed the
Blue Ridge and Black Log anticlines while they were yet buried; but its
channel-cutting was much retarded on encountering them, and some branch
stream working around from the lower side of the obstructions may have
diverted the river to an easier path. The only path of the kind is the
narrow one between the overlapping anticlines of Blue Ridge and Shade
mountains, and there the Juniata now flows. If another elevation should
occur in the future, it might happen that the slow deepening of the
channel in the hard Medina beds which now floor the Narrows would allow
Middle creek of Snyder county to tap the Juniata at Lewistown and lead
it by direct course past Middleburgh to the Susquehanna; thus it would
return to the path of its youth.
The location of the Juniata at the end of Tuscarora mountain is again
so definite that it can hardly be referred to a time when the mountain
had not been revealed. The most likely position of the original
cross-country stream which brought the Juniata into the Wiconisco
syncline was somewhere on the line of the existing mountain, and
assuming it to have been there, we must question how it has been
displaced. The process seems to have been of the same kind as that just
given; the retardation of channel-cutting in the late Cretaceous cycle,
when the Medina beds of Tuscarora anticline were discovered, allowed a
branch from the lower part of the river to work around the end of the
mountain and lead the river out that way. The occurrence of a shallow
depression across the summit of the otherwise remarkably even crest of
Tuscarora mountain suggests that this diversion was not finally
accomplished until shortly after the Tertiary elevation of the country;
but at whatever date the adjustment occurred, it is natural that it
should pass around the eastern end of the mountain and not around the
western end, where the course would have been much longer, and
therefore not successfully to be taken by a diverting stream.
While the quality of these processes appears satisfactory, I am not
satisfied as to the sufficiency of their quantity. If diversion was
successfully practiced at the crossing of the Tuscarora anticline, why
not also at the crossing of Jack's mountain anticline, on which the
river still perseveres. It is difficult here to decide how much
confidence may be placed in the explanation, because of its giving
reason for the location of certain streams, and how much doubt must be
cast upon it, because it seems impossible and is not of universal
application.
39. _Migration of the Atlantic-Ohio divide_.--There are certain shifted
courses which cannot be definitely referred to any particular cycle,
and which may therefore be mentioned now. Among the greatest are those
by which the divide between the Atlantic and the Ohio streams has been
changed from its initial position on the great constructional Nittany
highland and Bedford range. There was probably no significant change
until after Newark depression, for the branches of the Anthracite river
could not have begun to push the divide westward till after the
eastward flow of the river was determined; until then, there does not
seem to have been any marked advantage possessed by the eastward
streams over the westward. But with the eastward escape of the
Anthracite, it probably found a shorter course to the sea and one that
led it over alternately soft and hard rocks, instead of the longer
course followed by the Ohio streams over continuous sandstones. The
advantage given by the greater extent of soft beds is indicated by the
great breadth of the existing valleys in the central district compared
with the less breadth of those in the plateau to the west. Consider the
effect of this advantage at the time of the Jurassic elevation. As the
streams on the eastern slope of the Nittany divide had the shortest and
steepest courses to the sea, they deepened their valleys faster than
those on the west and acquired drainage area from them; hence we find
reason for the drainage of the entire Nittany and Bedford district by
the Atlantic streams at present. Various branches of what are now the
Alleghany and Monongahela originally rose on the western slope of the
dividing range. These probably reached much farther east in pre-Permian
time, but had their headwaters turned another way by the growth of the
great anticlinal divide; but the smaller anticlines of Laurel ridge and
Negro mountain farther west do not seem to have been strong enough to
form a divide, for the rivers still traverse them. Now as the
headwaters of the Juniata breached the eastern slope of the
Nittany-Bedford range and pushed the divide westward, they at last
gained possession of the Siluro-Devonian monocline on its western
slope; but beyond this it has not been possible for them yet to go. As
the streams cut down deeper and encountered the Medina anticline near
the core of the ridge, they sawed a passage through it; the Cambrian
beds were discovered below and a valley was opened on them as the
Medina cover wore away. The most important point about this is that we
find in it an adequate explanation of the opposite location of
water-gaps in pairs, such as characterize the branches of the Juniata
below Tyrone and again below Bedford. This opposite location has been
held to indicate an antecedent origin of the river that passes through
the gaps, while gaps formed by self-developed streams are not thought
to present such correspondence (Hilber). Yet this special case of
paired gaps in the opposite walls of a breached anticline is manifestly
a direct sequence of the development of the Juniata headwaters. The
settling down of the main Juniata on Jack's mountain anticline below
Huntingdon is another case of the same kind, in which the relatively
low anticlinal crest is as yet not widely breached; the gaps below
Bedford stand apart, as the crest is there higher, and hence wider
opened; and the gaps below Tyrone are separated by some ten or twelve
miles.
When the headwater streams captured the drainage of the Siluro-Devonian
monocline on the western side of the ancient dividing anticline, they
developed subsequent rectangular branches growing like a well-trained
grape vine. Most of this valley has been acquired by the west branch of
the Susquehanna, probably because it traversed the Medina beds less
often than the Juniata. For the same reason, it may be, the West Branch
has captured a considerable area of plateau drainage that must have
once belonged to the Ohio, while the Juniata has none of it; but if so,
the capture must have been before the Tertiary cycle, for since that
time the ability of the West Branch and of the Juniata as regards such
capture appears about alike. On the other hand, Castleman's river, a
branch of the Monongahela, still retains the drainage of a small bit of
the Siluro-Devonian monocline, at the southern border of the State,
where the Juniata headwaters had the least opportunity to capture it;
but the change here is probably only retarded, not prevented entirely;
the Juniata will some day push the divide even here back to the
Alleghany Front, the frontal bluff of the plateau.
[Illustration: FIG. 26.]
40. _Other examples of adjustments_.--Other examples of small
adjustments are found around the Wyoming basin, fig. 26. Originally all
these streams ran centripetally down the enclosing slopes, and in such
locations they must have cut gullies and breaches in the hard
Carboniferous beds and opened low back country on the weaker Devonians.
Some of the existing streams still do so, and these are precisely the
ones that are not easily reached by divertors. The Susquehanna in its
course outside of the basin has sent out branches that have beheaded
all the centripetal streams within reach; where the same river enters
the basin, the centripetal streams have been shortened if not
completely beheaded. A branch of the Delaware has captured the heads of
some of the streams near the eastern end of the basin. Elsewhere, the
centripetal streams still exist of good length. The contrast between
the persistence of some of the centripetal streams here and their
peripheral diversion around Broad Top is a consequence of the
difference of altitude of the old lake bottoms in the two cases. It is
not to be doubted that we shall become acquainted with many examples of
this kind as our intimacy with rivers increases.
41. _Events of the Quaternary cycle_.--The brief quaternary cycle does
not offer many examples of the kind that we have considered, and all
that are found are of small dimensions. The only capturing stream that
need be mentioned has lately been described as a "river pirate;"[23]
but its conquest is only a Schleswig-Holstein affair compared to the
Goth- and Hun-like depredations of the greater streams in earlier
cycles.
[Footnote 23: Science, xiii, 1889, 108.]
The character of the streams and their valleys as they now exist is
strikingly dependent in many ways on the relation of the incipient
quaternary cycle to the longer cycles of the past. No lakes occur,
exception being made only of the relatively small ponds due to drift
obstruction within the glaciated area. Waterfalls are found only at the
headwaters of small streams in the plateau district, exception again
being made only for certain cases of larger streams that have been
thrown from their pre-glacial courses by drift barriers, and which are
now in a very immature state on their new lines of flow. The small
valleys of this cycle are shallow and narrow, always of a size strictly
proportional to the volume of the stream and the hardness of the
enclosing rocks, exception being made only in the case of post-glacial
gorges whose streams have been displaced from their pre-glacial
channels. The terraces that are seen, especially on the streams that
flow in or from the glaciated district, are merely a temporary and
subordinate complication of the general development of the valleys. In
the region that has been here considered, the streams have been seldom
much displaced from their pre-glacial channels; but in the northwestern
part of the State, where the drift in the valleys seems to be heavier,
more serious disturbance of pre-glacial courses is reported. The facts
here referred to in regard to lakes, falls, gorges, terraces and
displaced streams are to be found in the various volumes of the Second
Geological Survey of the State;[24] in regard to the terraces and the
estuarine deflections of the Delaware and Susquehanna, reference should
be made also to McGee's studies.[25]
[Footnote 24: Especially Carll, Reports I_{3}, I_{4}; White, Reports
G_{5}, G_{6}; Lewis, Report Z.]
[Footnote 25: Amer. Journ. Science, xxxv, 1888, 367, 448; Seventh
Annual Rep. U. S. G. S., 1888, 545.]
42. _Doubtful cases_.--It is hardly necessary to state that there are
many facts for which no satisfactory explanation is found under the
theory of adjustments that we have been considering. Some will
certainly include the location of the Susquehanna on the points of the
Pocono synclines under this category; all must feel that such a
location savors of an antecedent origin. The same is true of the
examples of the alignment of water-gaps found on certain streams; for
example, the four gaps cut in the two pairs of Pocono and Pottsville
outcrops at the west end of the Wyoming syncline, and the three gaps
where the Little Schuylkill crosses the coal basin at Tamaqua; the
opposite gaps in pairs at Tyrone and Bedford have already been
sufficiently explained. The location of the upper North Branch of the
Susquehanna is also unrelated to processes of adjustment as far as I
can see them, and the great area of plateau drainage that is now
possessed by the West Branch is certainly difficult to understand as
the result of conquest. The two independent gaps in Tussey's mountain,
maintained by the Juniata and its Frankstown branch below Tyrone are
curious, especially in view of the apparent diversion of the branch to
the main stream on the upper side of Warrior's ridge (Oriskany), just
east of Tussey's mountain.
43. _Complicated history of our actual rivers_.--If this theory of the
history of our rivers is correct, it follows that any one river as it
now exists is of so complicated an origin that its development cannot
become a matter of general study and must unhappily remain only a
subject for special investigation for some time to come. It was my hope
on beginning this essay to find some teachable sequence of facts that
would serve to relieve the usual routine of statistical and descriptive
geography, but this is not the result that has been attained. The
history of the Susquehanna, the Juniata, or the Schuylkill, is too
involved with complex changes, if not enshrouded in mystery, to become
intelligible to any but advanced students; only the simplest cases of
river development can be introduced into the narrow limits of ordinary
instruction. The single course of an ancient stream is now broken into
several independent parts; witness the disjointing and diversion of the
original Juniata, which, as I have supposed, once extended from Broad
Top lake to the Catawissa basin. Now the upper part of the stream,
representing the early Broad Top outlet, is reduced to small volume in
Aughwick creek; the continuation of the stream to Lewistown is first
set to one side of its original axial location and is then diverted to
another syncline; the beheaded portion now represented by Middle creek
is diverted from its course to the Catawissa basin by the Susquehanna;
perhaps the Catawissa of the present day represents the reversed course
of the lower Juniata where it joined the Anthracite. This unserviceably
complicated statement is not much simplified if instead of beginning
with an original stream and searching out its present disjointed parts,
we trace the composition of a single existing stream from its once
independent parts. The Juniata of to-day consists of headwaters
acquired from Ohio streams; the lake in which the river once gathered
its upper branches is now drained and the lake bottom has become a
mountain top; the streams flow around the margin of the lake, not
across its basin; a short course towards Lewistown nearly coincides
with the original location of the stream, but to confound this with a
precise agreement is to lose the true significance of river history;
the lower course is the product of diversion at least at two epochs and
certainly in several places; and where the river now joins the
Susquehanna, it is suspected of having a superimposed course unlike any
of the rest of the stream. This is too complicated, even if it should
ever be demonstrated to be wholly true, to serve as material for
ordinary study; but as long as it has a savor of truth, and as long as
we are ignorant of the whole history of our rivers, through which alone
their present features can be rightfully understood, we must continue
to search after the natural processes of their development as carefully
and thoroughly as the biologist searches for the links missing from his
scheme of classification.
44. _Provisional Conclusions_.--It is in view of these doubts and
complications that I feel that the history of our rivers is not yet
settled; but yet the numerous accordances of actual and deductive
locations appear so definite and in some cases so remarkable that they
cannot be neglected, as they must be if we should adhere to the
antecedent origin of the river courses.
The method adopted on an early page therefore seems to be justified.
The provisional system of ancient consequent drainage, illustrated on
fig. 21, does appear to be sufficiently related to the streams of
to-day to warrant the belief that most of our rivers took their first
courses between the primitive folds of our mountains, and that from
that distant time to the present the changes they have suffered are due
to their own interaction--to their own mutual adjustment more than to
any other cause. The Susquehanna, Schuylkill, Lehigh and Delaware are
compound, composite and highly complex rivers, of repeated mature
adjustment. The middle Susquehanna and its branches and the upper
portions of the Schuylkill and Lehigh are descendants of original
Permian rivers consequent on the constructional topography of that
time; Newark depression reversed the flow of some of the transverse
streams, and the spontaneous changes or adjustments from immature to
mature courses in the several cycles of development are so numerous and
extensive that, as Löwl truly says, the initial drainage has almost
disappeared. The larger westward-flowing streams of the plateau are of
earlier, Carboniferous birth, and have suffered little subsequent
change beyond a loss of headwaters. The lower courses of the Atlantic
rivers are younger, having been much shifted from their Permian or
pre-Permian courses by Newark and Cretaceous superimposition, as well
as by recent downward deformation of the surface in their existing
estuaries. No recognizable remnant of rivers antecedent to the Permian
deformation are found in the central part of the State; and with the
exception of parts of the upper Schuylkill and of the Susquehanna near
Wilkes-Barre, there are no large survivors of Permian consequent
streams in the ordinary meaning of the term "consequent." The shifting
of courses in the progress of mature adjustment has had more to do with
determining the actual location of our rivers and streams than any
other process.
Harvard College, June, 1889.
TOPOGRAPHIC MODELS.
BY COSMOS MINDELEFF.
Of the many methods by which it has been sought to represent the relief
of a country or district, only two have been at all widely used. These
methods are, in the order of their development, by hachured and by
contoured maps. Both have advantages and both have serious
disadvantages. Without entering into the controversy that is even yet
raging over the relative merits of the two systems, some slight notice
of what each claims to accomplish is necessary.
The representation of relief by hachures is a graphic system, and in
the best examples we have is an attempt to show, upon a plane surface,
the actual appearance of a given area under given conditions of
lighting,--as in the Dufour map of the Alps. Of course certain details
that would really disappear if the assumed conditions were actual ones,
must be shown upon the map,--so that it is, after all, but a
conventional representation. The very best examples are, for this and
other reasons, unsatisfactory, and far more so is this the case in the
vastly larger class of medium grade and poor work.
The contour system represents relief by a series of lines, each of
which is, at every point throughout its length, at a certain stated
elevation above sea-level, or some other datum-plane; in other words,
each contour line represents what would be the water's edge, if the sea
were to rise to that elevation. It possesses the advantage of great
clearness, but fails to a large degree in the representation of surface
detail; moreover, one must have considerable knowledge of topography,
in order to read the map correctly.[1]
[Footnote 1: For specimens of representation of the same subject on
different scales, in both the hachure and contour systems, see plate
from "Enthoffer's Topographical Atlas."]
To those who must give first place to the quantity of relief rather
than the quality, as, for example, the geologist or the engineer, a
contoured map is now considered essential. On the other hand, where
quality of relief is the prime consideration and the quantity a
secondary one, as, for example, for the use of the army, a hachured map
is considered the best. The method of hachures may be roughly
characterized as a graphic system with a conventional element, and the
contour method as a conventional system with a graphic element,--for if
the contour interval is small enough a sort of shading is produced
which helps considerably the idea of relief.
In addition to these two great systems, with which everyone is more or
less familiar, there is another method of representing a country or
district,--a method that succeeds where others fail, and which although
by no means new, has not received the attention it deserves: this is
the representation of a country by a model in relief. Certain striking
advantages of models over maps of all kinds are, indeed, so apparent
that one almost loses sight of such slight disadvantages as can, of
course, be urged against them. In the graphic representation of the
surface they are far superior to the hachured map, and they have the
further advantage of expressing the relative relief, which the hachured
map fails to do, except in a very general way. They have also the
advantage of showing actual shadows, exactly as they would be seen in a
bird's-eye view of the district, instead of more or less conventional
ones, and are, consequently, more easily comprehended by the layman,
without becoming any less valuable to the skilled topographer. In
short, they combine all the graphic features of a hachured map with all
the advantages of the best class of contoured maps, and in addition
they show more of the surface detail, upon which so much of the
character of the country depends and which is very inadequately
expressed by hachures and almost completely ignored in a contoured map
of large interval. The contours themselves can be made to appear upon
the model very easily and without interfering with other features.
The uses of models are many and various. Within the past few years
their usefulness has been much extended, and, now that they are
becoming better known, will probably receive a still further extension.
To the geologist they are often of great value in working out the
structure of complicated districts, for the reason that so many
important structural relations can be presented to the eye at a single
glance. Similarly, for the graphic presentation of results there is no
better method, as the topography, the surface geology, and any number
of sections can be shown together and seen in their proper
relationship. To the engineer an accurate model is often of the
greatest assistance in working out his problems, and it is simply
invaluable to explain the details of a plan to anyone who has little or
no technical training; for, as has been stated, a model is easily
comprehended by anyone, while more or less technical knowledge is
required for the proper understanding of even the best maps.
I might go on cataloguing in detail the many uses to which models may
be put, but shall now mention only one more--perhaps the most important
of all--their use in the education of the young. No method has yet been
devised that is capable of giving so clear and accurate a conception of
the principles of physical geography as a series of well selected
models; models have, indeed, already been used for this purpose, but
unfortunately their great cost has prevented their general use in
schools. Since, however, the study of geography has been placed upon a
new basis and a new life has been infused into it, many men have given
their attention to the subject of models, and have experimented with a
view to cheapen the cost of reproduction, which has hitherto prevented
their wide distribution; and probably this objection will soon be
remedied. The ability to read a map correctly,--to obtain from a study
of the map a clear conception of the country represented,--is more
uncommon than is usually supposed. Some of the recent methods of
teaching geography are intended to cultivate this very faculty, but it
is doubtful whether there is any better method than that which consists
in the study of a series of good models in conjunction with a series of
maps, all on the same scale and of the same areas. The value of a
series of good models in teaching geology is so apparent that it need
only be mentioned. It is often, for reasons stated above, far more
valuable even, than field instruction.
For the construction of a good relief map the first requisite is a good
contoured map. To this should be added, when possible, a good hachured
map, upon which the elevations of the principal points are stated,--if
the interval in the contoured map is a large one,--and as much material
in the way of photographs and sketches as it is possible to procure.
The modeler should, moreover, have some personal acquaintance with the
region to be represented, or, failing that, a general knowledge of
topographic forms, and at least a clear conception of the general
character of the country which he seeks to represent. This is very
important, for it is here that many modelers fail: the mechanical
portion of the work any ordinarily intelligent person can do. A model
may be as accurate as the map from which it is made, every contour may
be placed exactly where it belongs, and yet the resulting model may
be,--indeed, often is--"flat," expressionless, and unsatisfactory.
Every topographer in drawing his map is compelled to generalize more or
less, and it is fortunate for the map if this be done in the field
instead of in the draughtsman's office. But topographers differ among
themselves: there may be, and often is, considerable difference in two
maps of the same region made by different men; in other words, the
"personal equation" is a larger element in a map than is usually
supposed. This being the case, there is something more required in a
modeler than the mere transferring of the matter in the map,--giving it
three dimensions instead of two: he must supply through his special
knowledge of the region (or, failing that through his general
knowledge) certain characteristics that do not appear upon the map, and
undo, so far as it is necessary, certain generalizations of the
topographer and draughtsman. This artistic or technical skill required
correctly to represent the _individuality_ of a given district is
especially important in the modeler; it is more important, perhaps, in
small-scale maps of large districts than in large-scale maps of small
ones,--for in the latter the generalizing process has not been carried
so far, and the smaller interval of the contour lines preserves much of
the detail.
The methods by which relief maps are made have always received more
attention than would, at first sight, appear to be their proper
proportion. It may be due, however, to the difficulty of applying any
test to determine the accuracy of the finished model, and perhaps also
to the general impression that any one can make a relief map,--and so
he can, though of course there will be a wide difference in the value
of the results. Some, indeed, have devoted their attention to methods
exclusively, letting the result take care of itself,--and the models
show it. There is no more reason why a modeler should tie himself down
to one method of work, than that a water-colorist, or a chemist, or
anyone engaged in technical work, should do so; though in some cases he
might be required, as the chemist is, to show his methods as well as
his results.
One of the earliest methods, with any pretension to what we may term
mechanical control, is that described by the Messrs. Harden in a paper
on "The construction of maps in relief," read before the American
Institute of Mining Engineers in 1887. The method was published in
1838. Upon a contoured map as a basis cross-section lines are drawn at
small and regular intervals, and, if the topography be intricate,
corresponding lines at right angles. The sections thus secured are
transferred to thin strips of some suitable material, such as
card-board or metal, and cut down to the surface line,--the strips
themselves thus forming the cross-sections. These cross-sections are
mounted upon a suitable base-board, and the cavities or boxes are then
filled up with some easily carved material, such as plaster or wax. The
top is then carved down to the form of the country or district,--the
necessary guidance being obtained by the upper edges of the strips that
form the cross-sections. It will be readily seen that this method is a
very crude and laborious one. It necessitates in the first place a good
contoured map upon which to draw the sections, but sacrifices much of
the advantage thus gained because only a number of points on each
contour line are used, instead of the entire line. It is no better,
although actually more laborious, than the later method of driving
contour pins (whose height above a base-board may be accurately
measured,) along the contour lines, and then filling in. A slight
modification of the latter method can be used to advantage when no
contoured map is available, and when the points whose elevation is
known are not numerous enough to permit the construction of one. In
this case the only control that can be secured is by means of a number
of pins driven into the base-board at those points whose elevation is
known. The remainder of the map is then sketched in. This method is
perhaps as satisfactory as any, when the material upon the map is
scanty. Another method, however, growing out of the same scantiness of
material, is in some cases to be preferred, especially for large
models. The map is enlarged to the required size, and a tracing of it
is mounted upon a frame. Another deep frame, just large enough to
contain the mounted tracing, is made, and laid upon a suitable
base-board upon which a copy of the map has been mounted. Upon this
base-board the model is then commenced, in clay or wax. The low areas
are modeled first,--horizontal control being obtained by pricking
through the mounted tracing of the map with a needle point, and
vertical control by measuring down from a straight edge sliding on the
top of the deep frame. This system is rather crude, and only useful
where the material upon the map is very scanty, but it gives excellent
control.
A method used by Mr. F. H. King in the preparation of his large map of
the United States is described by him in a letter to Messrs. Harden,
and published by them in the place mentioned. A solid block of plaster
is used,--the contoured map being transferred to it--and the plaster is
carved down to produce a series of steps like those made by building up
the contours. The shoulders are then carved down to produce a
continuous surface. This method is one of the best of those that
require carving instead of modeling.
Many other methods of producing relief maps might be mentioned, but, as
most of them have been used only to make special models, they need not
be described. The method that has been more used than any other still
remains to be described. It is that which the writer has used almost
exclusively, and consists in building up the model and modeling the
detail, instead of carving it. It is a maxim of the modeler that the
subject should be built up as far as possible, should be produced by
adding bits of clay or wax, or other material, and not by carving away
what is already on,--by addition and not by subtraction. This may be
illustrated by a reference to the methods of the sculptor. The bust, or
figure, or whatever the subject may be, is first modeled in clay or
wax; from this model a plaster mould is made, and from this mould a
plaster cast is taken. This cast is called the original, and the
finished production, whether in marble, bronze, or any other hard
substance, is simply a copy of this original. No one ever attempts to
produce the finished bust or figure directly from the object itself.
Even where the artist has for a guide a death mask, the procedure does
not change. The bust is first made in clay, and this clay model, as a
rule, contains all the detail which subsequently appears in the
finished bust. It seems strange, therefore, that the relief map maker
should use a method which the sculptor, with infinitely more skill and
judgment, is afraid to use; and this on subjects that do not differ as
much as might be imagined.
The contour interval to be used depends on the use to which the model
is to be put. It is not always necessary to carry into the model all
the contour lines upon the map: I may go further and say that it is not
always desirable to do so. The number to be used depends to some extent
on the skill of the modeler. As already stated, the contours are only a
means of control, and one modeler requires more than another. To build
into a model every contour in a contoured map of ten foot interval is a
very laborious proceeding, and not worth the time it takes, as in nine
out of ten maps of such interval only the fifty-foot or the one
hundred-foot curves are definitely fixed, the intermediate lines being
merely filled in. This filling in can be done as well, or better, by
the modeler.
The question as to the proper amount of exaggeration to be given the
vertical scale, as compared with the horizontal, is the question about
which has raged most of the controversy connected with relief map
making. This controversy has been rather bitter; some of the opponents
of vertical exaggeration going to the length of saying that no
exaggeration is necessary, and that "he that will distort or exaggerate
the scale of anything will lie." On the other hand the great majority
of those who have made relief maps insist upon the necessity of more or
less exaggeration of the vertical scale--generally more than seems to
me necessary, however.
An increase of angle of slope accompanies all vertical exaggeration,
and this is apparent even in models in which the vertical element is
only very slightly exaggerated. It produces a false effect by
diminishing the proportionate width of the valleys, and by making the
country seem much more rugged and mountainous than it really is. A
secondary effect is to make the region represented look very small--all
idea of the extent of the country being lost. This can be illustrated
better than described. The King model of the United States is an
example of one extreme; it is worthy of note that no examples of the
other extreme--too little exaggeration--are known.
In small-scale models of large districts some exaggeration of the
vertical scale is necessary in order to make the relief apparent, but
the amount of this exaggeration is often increased much beyond what is
essential. The proportion of scales must depend to a large extent on
the character of the country represented, and on the purposes for which
the model is made. It has been suggested by a writer, quoted by the
Messrs. Harden, that the following exaggeration would afford a pleasing
relief: "For a map, scale 6 inches to 1 mile: if mountainous, 1:3; if
only hilly, 1:2; if gently undulating, 2:3. For smaller scales, except
for very rugged tracts, the exaggeration should be correspondingly
increased. For a tract consisting wholly of mountains no exaggeration
is necessary." I know of no country of such a character that its
relief, in all its detail, cannot be shown upon a scale of 6 inches to
1 mile without any exaggeration at all.
It seems to me that the absolute and not the relative amount of relief
is the desideratum, and I have always used this as my guiding
principle. For small scale models I have found half an inch of relief
ample. It may be worth while to state that in a model of the United
States made for the Messrs. Butler, of Philadelphia, the horizontal
scale was 77 miles to 1 inch, the vertical scale 40,000 feet to 1 inch,
and the proportion of scales as 1 to 10. This proportion could have
been brought down as low as 1:6 with advantage. One-fortieth of an inch
to a thousand feet seems a very small vertical scale, but it sufficed
to show all the important features of the relief. It should be stated,
moreover, that the model in question was very hurriedly made--in fact,
was hardly more than a sketch-model--and that more care and more minute
work would have brought out many details that do not now appear. This
amount of care was not considered necessary in this instance, as the
model was made to be photographed and published as a photo-engraving,
and was to suffer an enormous reduction--coming down to five by seven
inches.[2]
[Footnote 2: See plate from "Butler's Complete Geography."]
It has been frequently urged by the advocates of large exaggeration
that the details of a country cannot be shown unless the vertical scale
is exaggerated; that hills 200, 300, or even 500 feet high--depending
of course upon the scale--flatten out or disappear entirely. This seems
plausible, but the advantages of great exaggeration are more apparent
than real. Its effect upon the model has already been mentioned; it
should be added that, with the proper amount of care in finishing the
model, exceedingly small relief can be so brought out as to be readily
seen. With ordinary care, one-fortieth of an inch can be easily shown,
and with great care and skill certainly one-eightieth and probably
one-hundredth of an inch. Another plausible argument that has been
advanced in favor of vertical exaggeration as a principle, is well
stated by Mr. A. E. Lehman, of the Pennsylvania Geological Survey, in a
paper on "Topographical Models," read before the American Institute of
Mining Engineers in 1885. "A perfectly natural expression is of course
desired; and to cause this the features of the topography should be
distorted and exaggerated in vertical scale just enough to produce the
same effect on the beholder or student of the district of country
exhibited as his idea of it would be if he were on the real ground
itself. Care should be taken, however, not to make the scales so
disproportionate as to do violence to mental impressions. Often,
indeed, prominent or important features, when they will bear it, may be
still more effectively shown by additional exaggeration in the vertical
scale." The fallacy of this argument is obvious. It assumes that the
object of a model is to show the country as it appears to one passing
through it, and not as it really is--and there is often a very wide
difference between the two. The impression derived from passing through
a country is, if I may use the term, a very large-scale impression, as
any one who has tried it can certify; it is certainly a mistake to
attempt to reproduce this impression in a small-scale model, with the
help of vertical exaggeration. Even if the principle were a good one,
its application would be very limited. It could only be used in
large-scale models; to apply it to a model of a large area--the United
States, for example--is obviously absurd.
The method referred to as being now generally in use may be briefly
described as follows: requisites, a good contoured map; a hachured map
in addition, if possible; a clear conception on the part of the modeler
of the country to be represented; and a fair amount of skill.
Materials: a base-board of wood or other suitable material; card-board
or wood of the thickness required by the contour interval and the
scale; and modeling wax or clay. Procedure: reproduce the contours in
the wood or other material; mount these upon the base-board in their
proper relationship; then fill in the intervening spaces, and the space
above the topmost contour, with the modeling material.
In a series of models of the Grand Divisions of the earth, made about a
year and a half ago, the contours of card-board were made as follows:
the map was photographed up to the required scale, and as many prints
were made as there were contour intervals to be represented--in a model
of the United States of 1,000 feet contour interval there were fourteen
prints. Thirteen of these were mounted upon card-board of the exact
thickness required by the vertical scale, and one upon the base-board.
All large paper companies use a micrometer gauge, and card-board can
easily be obtained of the exact thickness required--even to less than
the thousandth part of an inch. The lowest contour was then sawed out
upon a scroll saw, and placed upon the corresponding line of the map
mounted upon the base-board. This process was repeated with each of the
succeeding contours until all were placed and glued into their proper
positions. At this stage the model presents the relief in a series of
steps, each step representing a rise corresponding to the contour
interval. The disadvantages of the method lie in the fact that unless
the greatest care is exercised in making the photographic prints there
will be considerable distortion, owing to the stretching of the paper
in different directions, and consequently much trouble in fitting the
contours. If care be exercised in having the grain of the paper run in
the same direction in all the prints, trouble in fitting the contours
will be much reduced, but the distortion in one direction will remain.
In our experience this distortion amounts to about two per cent.; in
other words, a model that should be fifty inches long will in reality
be fifty-one inches; but, as this error is distributed over the whole
fifty inches, it is not too great for an ordinary model. If greater
accuracy be required, it can be secured by transferring the contours to
the card-board by means of tracing or transfer paper. The great
advantage of the photographic method lies in the fact that when the
model has been built up, with all the contours in position, it presents
a copy of the map itself, with all the details, drainage, etc., in
position, instead of blank intervals between the contours. Such details
and drainage are a great help in the subsequent modeling.
The next step in the process is to fill in with clay or wax the
intervals between the contours. I have always found wax more convenient
than clay for this purpose as, unless the surface coating is a thick
one, the clay is difficult to keep moist. To obviate this difficulty,
some modelers have used clay mixed with glycerine instead of water;
this, of course, does not become dry, but the material is, at its best,
unsatisfactory. The filling-in process is the most important one in
relief map making, for it is here that the modeler must show his
knowledge of, and feeling for, topographic forms. Some models seem to
have been constructed with the idea that when the contours have been
accurately placed the work of the modeler is practically done. This is
a great mistake. The card-board contours are only a means of control,
occupying somewhat the same relation to the relief map that a core or
base of bricks, or a frame of wood, does to other constructions as, for
example, an architectural ornament or a bust. It is sometimes necessary
to cut away the contour card; for, as has been already explained, a map
is more or less generalized, and a contour is frequently carried across
a ravine, instead of following it up, as it would do if the map were on
a larger scale. Such generalizing is of course perfectly proper in a
map, but, with the same scale, we expect more detail in a model. The
modeler must have judgment enough and skill enough to read between the
lines, and to undo the generalizing of the topographer and draughtsman,
thus supplying the material omitted from the map. This can be done
without materially affecting the accuracy of the model, considered even
as a copy of the contoured map.
The contours of card-board or other material are, let me repeat, only a
means of control. The perfect modeler--a variety, by the way, yet to be
evolved--would be able to make an accurate relief map without them, in
the same way that other subjects are made; as, for example, a flower
panel, an architectural ornament, or any other subject in low relief,
where the object sought is artistic effect and great accuracy is not a
desideratum. It is the converse of this idea that has produced the
numerous models that one sees; accurate enough, perhaps, but wholly
expressionless and absolutely without feeling. This is the great fault
of nearly all models made by building up the contours in wood and then
carving down the shoulders. It is then necessary to sand-paper them,
and what little character they might otherwise have had is completely
obliterated by the sand-paper. Such models almost invariably _look_
wooden. Let the modeler, then, have a clear conception of his subject
and not depend wholly on the contours, and let him work out that
conception in his model, "controlled" and helped by the contours, but
not bound by them; the resulting model will thus be far more
satisfactory and a far better representation of his subject, in other
words, it will be more life-like--more nearly true to nature.
The model, provided it be not of clay, is sometimes used in the state
in which it is left when finished. It is much more common, however, to
make a plaster mould, and from this a plaster cast. For this purpose a
moulder is usually called in; but moulders as a rule are ignorant men,
accustomed to one line of work only, and the result is not always
satisfactory. It is much better for the modeler himself to do this
work, though to obtain good results from plaster it is necessary to
know the material thoroughly, and this knowledge comes only from
experience. The mould is generally made quite heavy, in order to stand
the subsequent hard treatment that it may receive, and should be
retouched and thoroughly dried before being prepared for the cast. The
method used by some modelers of placing a frame about the model and
pouring in the plaster, filling the frame to the top, is a crude and
very wasteful one and not at all to be recommended. In a model of large
size--say seven or eight feet square--it would require a derrick to
move the mould. It is wholly unnecessary, as, with a small amount of
care, a good mould can be made not more than an inch thick, or, at
most, an inch and a half. The drying of the mould before use can
sometimes be dispensed with, but is always desirable.
Nearly all American moulders (as distinguished from French and Italian
ones) varnish the mould, and thus lose some of the finest detail and
sharpness. This is unnecessary. The mould can be easily prepared with a
solution of soap so as to leave nothing on the surface but a very thin
coating of oil, which is taken up and replaced by the plaster of the
cast. Of course, if the model has been sand-papered, no fine work in
moulding or casting is necessary, as there is nothing to save. If the
subject is a very intricate one, with "undercuts" (as they are called),
it is customary to make a waste mould; as this is very seldom necessary
in relief map work, however, the process need not be described.
To make the cast it is only necessary to repeat the processes used in
making the mould. With great care and some skill a cast can be produced
but little inferior in point of sharpness and detail to the original
model. It is customary to make the cast very thick, and, consequently,
very heavy; this is unnecessary. In our work we seldom make a cast
thicker than one inch, and yet are never troubled with changes in the
model after it is finished. Even in a very large cast (now in the
National Museum), weighing nearly 1,500 pounds and presenting a surface
of over 160 square feet, the average thickness is less than one inch,
although it required over five barrels of plaster to make it. The cast,
after being thoroughly dried, should be finished--all its imperfections
being carefully repaired. The surface, however, should be touched as
little as possible, as the slight roughness of surface that comes from
the original model, through the mould, is removed by any tooling. This
roughness adds much to the effect of the model; in fact, where the
scale is large enough, it is sometimes desirable to emphasize it.
The proper way to paint a model is a matter that must rest principally
upon the judgment of the modeler, depending to some extent, also, on
the use to which the model is to be put. The plain cast is sometimes
used, drainage, lettering, etc., being put directly upon it. This has
the advantage of preserving all the detail that comes from the mould,
but it has also the disadvantage of a surface easily soiled and
impossible to clean. If the model is to be photographed, the surface
should be nearly white--in our practice we use a small amount of yellow
with the white. This yellow is hardly appreciable by the eye, but its
effect upon the photographic negative is quite marked. Yellow becomes
grey in a photograph, and, in a photograph of a model colored as
described, a grey tint is given to the whole surface. The high lights
are not pure white, and there is no harsh contrast between light and
shade. There is another point of great importance in photographing
models: the surface should have a dead finish--that is, should have no
gloss, or, at most, should have only what is known among painters as an
egg-shell gloss. It is almost impossible satisfactorily to photograph a
model that has a shiny surface. Any portion of a model that it is
desired to separate from the rest should be painted a different
color--the water, for example, should be painted a light blue; not a
blue composed of indigo, however, or any of the grey blues, as these
produce in the photograph a dead grey, and are not pleasant to the eye.
The most satisfactory color that we have used is a mixture of
cobalt--the purest of the blues--with Antwerp blue--which is quite
green--and white. This gives a color that is pleasant to the eye, has
the retreating quality to perfection, and photographs well.
Models intended for exhibition as such should be painted realistically.
There is room here for an immense improvement in the usual practice,
which is to paint the model either in some conventional scheme of light
and shade, or else to put a single flat tint upon it. If the model is
to be colored conventionally it is, in my opinion, much better to use a
flat tint, light in tone, and with a dead surface. The use of a variety
of colors upon the face of a model interferes materially with the
relief, especially if the relief is finely modeled. For this reason
models colored to indicate geologic formations should always be
accompanied by duplicates representing topography only, colored
realistically, if possible, and without lettering. Well-defined lines
other than those pertaining to the model itself, such, for example, as
those used to define the boundaries of geologic formations, should not
be allowed upon a model when it is desired to bring out all the relief.
The lettering on such models should be kept down as small as possible,
or wholly dispensed with. The latter is much the better method.
The cheap reproduction of models is the most important problem
connected with the art, and the one that is attracting most attention
among those engaged in it; as, until models can be reproduced cheaply,
they will never have any wide distribution and there will be far less
incentive to the modeler. Various materials have been suggested and
experimented on, but nine-tenths of the models that are made to-day are
made of plaster of Paris. Although this material was the first to be
used for this purpose, it has not yet been superseded. A plaster cast
is heavy, expensive and easily injured; but plaster gives an accurate
copy of the original, retains permanently the form given it, and is
easily finished and repaired. The weight is an obstacle that can be
easily overcome. By the incorporation in the plaster of fine tow, or of
bagging or netting of various kinds, the cast can be made very light
and at the same time strong, but the expense is increased rather than
diminished by this method. Models made in this way, however, have the
advantage that when broken the pieces do not fall out, they are,
however, fully as liable to surface injury as the other kind. The large
cast in the National Museum, before referred to, was made in this way.
It weighed nearly 2,000 pounds when boxed--not an easy thing to
handle--but it stood shipment to New Orleans and back without suffering
any material injury. This would hardly have been possible had the cast
been made from plaster alone.
Paper seems, at first sight, to be the material best adapted for the
reproduction of models; but no one has succeeded well enough with it to
bring it into use. Like nearly all those who have given this subject
attention, I have experimented with paper, but the only positive result
has been a loss of a large part of the confidence that I once had in
the suitability of the material. Paper has been used extensively for
large scale models of pueblos, ruins, etc., but I have never obtained a
satisfactory result with subjects in low relief and fine detail. A
paper cast may look well when first made, but it absorbs moisture from
the atmosphere, and contracts and expands with the weather. The
contraction is apt to flatten out the model and the expansion to make
it buckle up.
Casts of models have been made in iron; but this, while suitable
perhaps for models of mounds and subjects of like character, would
hardly be applicable to small scale models with fine detail; such casts
require too much surface finishing. The material known as
Lincrusta-Walton seems to me to be the ideal material for this purpose.
It is tougher than rubber, will take the finest detail, and its surface
can be treated in any way desired. Unfortunately the manufacture of
models in this material would require expensive machinery, and is
outside the scope of a modeling room. Should it ever become
commercially advantageous, however, casts of a model of ordinary size,
in every way equal to the original, can be turned out in this material
at a very small cost.
It remains to speak of the reproduction of models by
process-engravings--a method that will probably receive much more
attention in the future than it has in the past. It is perhaps along
this line that the cheap reproduction of models will develop; but the
subject is too large a one to be adequately treated here, and must be
postponed until some future occasion.
[Illustration: HACHURED AND CONTOURED MAPS.
REPRESENTATION OF A HILL ACCORDING TO THE TWO SYSTEMS AND ON DIFFERENT
SCALES.
From Supplement to Enthoffer's Topographical Atlas by permission of Mr.
Enthoffer.]
[Illustration: FROM BUTLER'S COMPLETE GEOGRAPHY.
COPYRIGHT, 1888, BY E. H. BUTLER & CO.
Printed by permission.]
NATIONAL GEOGRAPHIC SOCIETY.
ABSTRACT OF MINUTES.
_October 5, 1888, Ninth Meeting_.
A paper was read entitled, "Topographic Models," by Mr. Cosmos
Mindeleff. Published in the "National Geographic Magazine," Vol. I, No.
3.
_October 19, 1888, Tenth Meeting_.
The attendance being very small, no paper was read.
_November 2, 1888, Eleventh Meeting_.
The paper of the evening was entitled, "Surveys, their Kinds and
Purposes," by Mr. Marcus Baker. The paper was discussed by Messrs.
Ogden, Goodfellow, Gannett and Baker. Published in "Science," Vol. XII,
No. 304.
_November 16, 1888, Twelfth Meeting_.
A paper was read by Mr. Henry Gannett, giving certain "Physical
Statistics Relating to Massachusetts," derived from the map of that
State recently prepared by the United States Geological Survey. A
discussion followed which was participated in by Messrs. Baker,
Kenaston, Fernow, Weed, and the author. A second paper entitled,
"Something about Tornadoes," was read by Lieut. J. P. Finley, U. S.
Signal Corps.
_November 30, 1888, Thirteenth Meeting_.
The annual reports of vice-Presidents Herbert G. Ogden and Gen. A. W.
Greely were delivered. Published in the "National Geographic Magazine,"
Vol. I, No. 2.
_December 20, 1888, Fourteenth Meeting_.
Held in the Law Lecture Room of the Columbian University. The President
delivered his Annual Address, entitled, "Africa." Published in the
"National Geographic Magazine," Vol. I, No. 2.
_December 28, 1888, Fifteenth Meeting_.
The Society met in the Society Hall of the Cosmos Club, President
Hubbard in the chair. Owing to the absence from the city of the
Secretaries, Mr. O. H. Tittmann was requested to act as Secretary of
the meeting. The minutes of the first and fourteenth meetings were read
and approved. The report of the Secretaries was read, in their absence,
by the temporary Secretary, and was approved. The Treasurer's report,
showing a balance on hand of $626.70, was read and approved, as was
also that of the auditing committee.
The President announced that vacancies caused by the resignation of two
of the managers, Messrs. W. D. Johnson and Henry Mitchell, had been
filled by the Board on the 15th of November, by the election of Messrs.
O. H. Tittmann and C. A. Kenaston; and that a vacancy caused by the
resignation of Vice-President John R. Bartlett, had been filled by the
election of Lieut. George L. Dyer, on November 30th.
The Society then proceeded to the election of officers, with following
result:
_President_--GARDINER G. HUBBARD.
_Vice-Presidents_--HERBERT G. OGDEN, [land]; GEORGE L. DYER, [sea];
A. W. GREELY, [air]; C. HART MERRIAM, [life]; A. H. THOMPSON,
[art].
_Treasurer_--CHARLES J. BELL.
_Recording Secretary_--HENRY GANNETT.
_Corresponding Secretary_--GEORGE KENNAN.
_Managers_--CLEVELAND ABBE, MARCUS BAKER, ROGERS BIRNIE, JR.,
G. BROWNE GOODE, W. B. POWELL, J. C. WELLING, C. A. KENASTON,
O. H. TITTMANN.
_January 11, 1889, Sixteenth Meeting_.
The paper of the evening was entitled, "The Great Plains of Canada,"
and was presented by Professor C. A. Kenaston, of Howard University.
_January 25, 1889, Seventeenth Meeting_.
The paper of the evening was entitled, "Irrigation in California," by
Mr. William Hammond Hall, State Engineer of California. To be published
in the "National Geographic Magazine," Vol. I, No. 4.
_February 8, 1889, Eighteenth Meeting_.
The following papers were read by Prof. W. M. Davis, of Harvard
University: "Topographic Models," and "Certain Peculiarities of the
Rivers of Pennsylvania." Published in the "National Geographic
Magazine," Vol. I, No. 3.
_February 22, 1889, Nineteenth Meeting_.
The paper of the evening was entitled, "Round about Asheville, N. C.,"
by Mr. Bailey Willis. The paper was illustrated by charcoal sketches
and lantern slides. Discussion followed, which was participated in by
Messrs. Baker, Merriam and McGee. To be published in the "National
Geographic Magazine," Vol. I, No. 4.
_March 8, 1889, Twentieth Meeting_.
The following amendments to the By-Laws were adopted.
[For Article VI substitute the following]:
ARTICLE VI.
MEETINGS.
"Regular meetings of the Society shall be held on alternate Fridays,
from November until May, and excepting the annual meeting, they shall
be devoted to communications. The Board of Managers shall, however,
have power to postpone or omit meetings, when deemed desirable. Special
meetings may be called by the President.
"The annual meeting for the election of officers shall be the last
regular meeting in December.
"The meeting preceding the annual meeting shall be devoted to the
President's annual address.
"The reports of the retiring Vice-Presidents shall be presented at the
meetings in January.
"A quorum for the transaction of business shall consist of twenty-five
active members."
In Article V, the following paragraph was introduced immediately after
the first paragraph of the article:
"The dues of members elected in November and December shall be credited
to the succeeding year."
The following papers were then presented: "A Trip to Panama and
Darien," by Mr. R. U. Goode, and "Survey of Mason and Dixon's Line," by
Mr. Mark B. Kerr.
A Trip to Panama and Darien, to be published in the "National
Geographic Magazine," Vol. I, No. 4.
_March 22, 1889, Twenty-first Meeting_.
The paper of the evening was entitled, "Recent Events in the U. S. of
Columbia," by Mr. W. E. Curtis. The discussion which followed was
participated in by Messrs. Baker, Gannett, and others.
_April 5, 1889, Twenty-second Meeting_.
The paper of the evening was entitled, "House Life in Mexico," by Mr.
A. B. Johnson.
_April 19, 1889, Twenty-third Meeting_.
This meeting was devoted to papers upon the Samoan Islands. The
following programme was presented:
"Samoa; the General Geography and Hydrography of the Islands and
Adjacent Seas," by Mr. Everett Hayden.
"Climate," by Prof. Cleveland Abbe.
"Narrative of a Cruise Among the Islands," by Capt. R. W. Meade, U. S.
N.
"The Home Life of the Samoans and the Botany of the Islands," by Mr. W.
E. Safford, U. S. N.
_May 3, 1889, Twenty-fourth Meeting_.
The paper of the evening was entitled, "Across Nicaragua with Transit
and Machéte," by Mr. R. E. Peary, U. S. N. To be published in the
"National Geographic Magazine," Vol. I, No. 4.
_May 17, 1889, Twenty-fifth Meeting_.
The paper of the evening was entitled, "The Krakatoa Eruption," by Dr.
A. Graham Bell. The paper was discussed by Captain C. E. Dutton.
(Translated by Mr. R. L. Lerch.)
INTERNATIONAL LITERARY CONTEST
To be held at Madrid, Spain, under the auspices of the Commission in
charge of the celebration of the Fourth Centennial Anniversary of the
Discovery of America.
PROGRAM.
The work for which a prize is offered is to be a prose essay, a true
historic picture giving a just estimate of the grandeur of the occasion
to be celebrated.
So much has been written on this subject since the opening of the XVIth
century that it would seem difficult to say anything new and good.
Perhaps the details, perhaps the circumstances in the life and acts of
Columbus are worthy of no little research; but already the Royal
Academy of History is engaged in the erudite and diligent task of
bringing together and publishing the un-edited or little known papers
bearing on this question.
The book required by this contest must be of a different nature: it
must be comprehensive and synoptic, and must be sufficiently concise
without being either obscure or dry.
Although there is an abundance of histories of America, of voyages and
discoveries, of geographic science, and of the establishment of
Europeans in remote regions of the earth, there is no book that sets
forth as it can be done the combined efforts of the nations of the
Iberian peninsula, who, since the commencement of the XVth century,
have, with a fixity of purpose and marvelous tenacity, in almost a
single century of silent efforts brought about the exploration of vast
continents and islands, traversed seas never before cut by Christian
prows, and in emulous strife obtained almost a complete knowledge of
the planet on which we live.
There is a growing interest and manifest unity in all those more
important events; not to mention the circumstantial evidence borne by
the charts of 1375 and the semi-fabulous voyages, such as that of Doria
y Vivaldi and others less apocryphal though isolated and barren of
results, like that of Ferrer, begun in 1434, when Gil Eannes doubled
Cape Bojador, discovered Guinea, and dispelled the terror inspired by
the unknown ocean, and ended in 1522 with Elcano's arrival at Sanlucar
after circumnavigating the globe.
In all this activity very little occurs by chance. The progressive
series of geographic discoveries, due to persistent premeditation and
not to accident, was inaugurated at Sagres by the Infante D. Enrique
and his illustrious pilot Jaime de Mallorca.
Well might Pedro Nuñes exclaim that from that time forth until the form
and size of the terraqueous globe were thoroughly known, the most to be
obtained would not be firmly established, "unless our mariners sailed
away better instructed and provided with better instruments and rules
of Astronomy and Geography than the things with which cosmographers
supplied them."
The culmination in the progress of that beautiful history falls on the
12th of October, 1492, when Columbus was the first European to set foot
upon the intertropical shores of the New World. But this act,
considered apart from its intrinsic value, as purely the individual
inspiration of a mariner and the generous enthusiasm of a patron Queen,
derives a higher value when regarded as part of a summation of efforts,
a grand development of an idea, a purpose to explore and know the whole
globe, to spread the name and the law of Christ together with the
civilization of Europe, and to reap a harvest of gold, spices, and all
the riches of which costly samples and exaggerated reports were
furnished by the traffic of the Venetians, Genoese and Catalonians, who
in turn got them from Mussulmans.
Doubtless the moving cause, whose gorgeous banner so many men of our
peninsula followed, was clothed in great sentiments, good or bad; their
hearts were filled with religious fervor, thirst for glory, ambition,
Christian love, cupidity, curiosity, and violent dissatisfaction (even
during the Renaissance), to seek and undergo real adventures that
should surpass the vain, fruitless, and fanciful adventures of
chivalry; and to make voyages and conquests eclipsing those of the
Greeks and Romans, many of which, recorded in classic histories and
fables, were now disinterred by the learned.
What must be described is the complete picture in all its sumptuousness
so that its magnificent meaning may stand out distinctly, without which
the conviction would be lacking that the studies, voyages, and happy
audacity of Bartolomé Diaz, Gama, Alburquerque, Cabral, Balboa,
Magallanes, Cortes, Pizarro, Orellana, and a host of others, do not dim
the glory of the hero whose centennary is to be celebrated, even though
it heighten and add greater luster to the work of civilization begun by
Portugal....
The book here vaguely outlined must also contain a compendious
introduction, notices of voyages, ideas, and geographic progress up to
the date of D. Enrique's establishment at Sagres, and an epilogue or
conclusion of greater extent, in which are examined and weighed the
changes and progress that our subject has made, collectively, in the
civilization of the world--in the commerce, economics and politics of
the peoples, in regard to the broad field opened to the intelligent
activity of Europe, over which it could spread and dominate; the
abundance of data, sunken hopes, and more secure basis lent to the
studious and wise for the extension of our knowledge of Nature, the
unraveling of her laws, and penetration of her mysteries.
The vast, elevated argument of the book requires it be a finished work
of art, not in fullness and richness of diction, but in plan and order,
in sobriety and unity of style, whose nobility and beauty must lie in
simplicity of phrase, correctness of judgment and richness of thought.
There may enter into this contest any unpublished work written to this
end in Spanish, Portuguese, English, German, French or Italian.
The tribunal that is to award the prize will be composed of two members
of the R. Acad. of History, and one member from each of the Spanish R.
Academies of Moral Sciences and Politics, and Exact and Natural
Sciences--all to be chosen by the Academies themselves.
Furthermore, there will be included in the tribunal the diplomatic
representative of every power whose subject or subjects wish to enter
the contest, which is to be done through said representative or some
person duly appointed to act in his place.
The tribunal will elect its presiding officer and will decide on the
best works by an absolute majority of all the jurors who take part in
the vote.
Each work submitted in this contest must be neatly copied, in legible
writing, on good paper, without the author's name but with a quotation
to identify him afterwards.
Each author will inclose a separate folded sheet on whose exterior is
written the quotation he has chosen and the opening sentence of his
work; within, he will write his name and residence.
The folded sheets corresponding to the works that did not get a prize
will be burnt publicly without being opened.
Though it is difficult to set a limit as to size, the works should not
have more reading matter than is contained in two volumes of the shape
and size of the complete works of Cervantes issued by Rivadeneyra in
1863-4.
If the plan or purpose of any of the works require it, there may be
added another volume of documents, maps, or other illustrations.
As it will take time to examine and judge the works, they should be
sent to the Secretary of the R. Acad. of Hist. prior to January 1,
1892.
There will be first prize of 30,000 pesetas ($5,790) and a second of
15,000 pesetas ($2,895).
Besides this, each of the two successful authors will receive 500
copies of the printed edition of his work.
It rests with the Centennial Commission to determine the number of
copies in the edition of each of the two prize works, and what
disposition is to be made of the copies that are not given to the
authors.
These (the authors) keep the right to re-print and to sell their works,
and to translate them into other tongues.
The Commission, however, will have the right, if either or both prize
works are in a foreign tongue, to have them translated and published in
Castilian.
The Commission affix their seal to the preceding directions for the
information of the public and government of those persons who desire to
participate in the contest.
Madrid, June 19, 1889.
The Vice President, DUKE OF VERAGUA.
Secretaries, JUAN VALERA, JUAN F. RIAÑO.
End of the Project Gutenberg EBook of The National Geographic Magazine, Vol.
I., No. 3, July, 1889, by Various
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