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Title: Problems of the Libyan Desert
Author: John Ball
Release date: October 20, 2025 [eBook #77091]
Language: English
Original publication: London: The Royal Geographical Society, 1927
Credits: Galo Flordelis (This file was produced from images generously made available by the HathiTrust Digital Library)
*** START OF THE PROJECT GUTENBERG EBOOK PROBLEMS OF THE LIBYAN DESERT ***
[Transcriber's note: This article has been extracted and prepared from
_The Geographical Journal_, v. 70, 1927.]
PROBLEMS OF THE LIBYAN DESERT
John Ball, O.B.E., D.Sc., M.Inst.C.E., F.G.S., Director of Desert
Surveys, Egypt
_Map following p._ 96.
THERE can be few tracts of the Earth’s surface which present such a
number and variety of interesting geographical problems as the Libyan
Desert. The following are some of the more puzzling of the questions
which have been asked at various times since I made my first
acquaintance with the desert thirty years ago:
1. Did the Nile, or a branch of it, ever flow through the Libyan
Desert to the west of the present Nile Valley?
2. By what natural agency were the great depressions in the Libyan
Desert excavated?
3. Can other depressions in this region, besides the much-discussed
Wadi Rayan, be considered possible of utilization in connection with
irrigation, flood-protection, or drainage of the Nile Delta, or as
sources of water-power for Egypt?
4. Whence comes the artesian water of the oases?
5. Why have not certain lakes, such as Sittra and Bahrein, situated as
they are in an almost rainless region, long since dried up?
6. Can the present scanty supplies of drinkable water derived from
local rainfall along the Egyptian Mediterranean Littoral be
supplemented by artesian borings?
7. Are the artesian water-supplies of the oases gradually diminishing?
8. Where shall we look for the mysterious “Zerzura,” or “Oasis of the
Blacks”? Are there any other “lost” oases remaining to be discovered?
9. Can the present difficulties of travel in the Libyan Desert, which
are chiefly due to scarcity of water, be alleviated by the sinking of
new wells?
10. Where are the “Tortoise Marshes” of Ptolemy?
11. What determined the peculiar distribution of the sand-dunes, some
of which extend in straight lines for hundreds of kilometres?
12. What is the full extent of the distribution of flint implements
and pottery, of which accumulations have been found in what are now
among the most desolate parts of the desert, and what light does this
throw on human history and climatic changes?
To most of these questions no completely satisfactory answers have as
yet been given, though several of them have been the subject of
painstaking investigations by many workers. Our knowledge of the facts
bearing on them has, however, increased considerably during the past few
years; and in the present paper I propose briefly to review the various
questions in the light of the latest available data concerning them.
The problems, though they concern a variety of subjects, are very much
interwoven with each other, and they are mostly alike in that the first
difficulty in any attempt to solve them has always been the
incompleteness of our geographical knowledge concerning the Libyan
Desert as a whole. If we possessed a series of detailed contour-maps
covering the whole of the desert, most of the problems would be in a
fair way of solution. But the surveys which would be necessary for such
a result are impracticable at present, mainly on the score of expense.
Some fifteen years ago I essayed the construction of a contoured map of
the Libyan Desert, in connection with the International “Million” map of
the world. But I had to give up the task, finding that there were vast
areas without a single observation for altitude, while such altitude-
data as did exist were open to errors of a magnitude intolerable for
even a preliminary contouring of the areas with which they were
concerned. The attempt however was not without its value, because it
drew attention to the great lack of reliable levels in the Libyan
Desert, and to the necessity of remedying this defect before even an
approximately true picture of the general relief could be obtained.
In the interval which has elapsed since my former attempt, a great deal
of surveying has been carried out in the Libyan Desert, and in most of
this work special attention has been paid to the matter of levels. Two
long chains of triangulation have been run westward from the Nile to
Siwa Oasis, one _viâ_ the Wadi Natrun and the other _viâ_ Baharia Oasis;
another chain has been completed along the coast westward from
Alexandria to Sollum, and a triangulation is now being carried out which
will eventually connect the oases of Kharga and Dakhla with the Nile
Valley. Small local triangulations have also been made in certain areas,
such as Dakhla Oasis and in the neighbourhood of Owenat. In all these
triangulations vertical as well as horizontal angles have been observed,
so that reliable trigonometric altitudes are now available for almost
all the points occupied or sighted. In the southern and central parts of
the desert, we are still dependent on barometric levels; but the old
determinations have mostly been replaced by later and better ones; and
the use of motor-car transport, by facilitating exploratory journeys in
hitherto untrodden regions, has permitted of a large number of
additional determinations being made in tracts where no previous
observations of any kind existed.[1]
Owing to the methods employed in the latest barometric determinations,
the resulting altitudes are of a much higher order of accuracy than
could be hoped for in previous measurements of the kind; in fact, it is
believed that they are not so very far behind trigonometric levels in
precision. This increased accuracy has been rendered possible by the
collaboration of the Egyptian Meteorological Service, which, thanks to
the abundant weather-data now received and dealt with by it, is in a
position to furnish a close approximation to the sea-level pressure at
any point in the deserts at any given instant. The aneroids used in the
desert explorations are compared with the standard barometer of the
Meteorological Service immediately before the start and immediately
after the return of each expedition. At each observation in the desert,
the date and time, and the temperature of the air, are noted, as well as
the approximate latitude and longitude of the place. On the return of
the expedition, the recorded reading is corrected for the error of the
aneroid on the standard barometer, and the Meteorological Service is
asked to supply the sea-level pressure at the particular place and time
of the observation. From the difference of the two pressures and the air
temperature, the height of the place above sea-level is then worked out
directly with the aid of Jordan’s table of “Barometrische Höhenstufen.”
If the place of observation lies considerably above sea-level, a small
correction is applied to the observed air-temperature to allow for the
temperature-gradient in the air; for of course it is the mean
temperature of the air column between the place and sea-level which
should be employed in the reduction.
It is unlikely that with this procedure either the sea-level pressures
or those observed at the places can be in error by much more than half a
millimetre; and consequently, even with some slight uncertainties in the
air-temperatures, the resulting altitudes may be expected to be accurate
to within about 15 metres when only a single observation has been made,
and to within much less than this when, as is usually the case at
important points, the altitudes have been calculated from a series of
readings spread over several days. That this degree of accuracy is
actually attained seems evident on comparison of the levels obtained for
the same places on different dates. Thus, for instance, at a camp close
to Bir Terfawi, during a stay of five days in January and two days in
March 1925, I made in all eighteen aneroid observations. The mean
altitude found was 244 metres; the average deviation of a single
observation from this mean was 8 metres, the maximum being 16. On the
assumption that all sources of error are accidental, the probable error
of the mean resulting altitude works out at less than 2 metres, which is
about the same as might be expected in trigonometric levelling to the
place from the Nile Valley.
The points in the interior of the Libyan Desert of which the levels are
fairly accurately known now number many hundreds, and with the exception
of a large blank in the unexplored west, and another between the Nile
and Merga in the Northern Sudan, they are fairly well distributed over
the country. It therefore seemed to me to be worth while to make another
attempt at a contoured map; for the levels now known (though still far
too few for contours to be drawn with any great precision) might suffice
for the construction of a map on a comparatively small scale and with
contours at fairly wide intervals; it was felt that a provisional map of
this kind, besides affording at least an approximate general view of the
relief, might help towards the solution of one or more of the problems
which I have mentioned above. I commenced by sketching in the contours
at 100-metre intervals on the original sheets of the 1:500,000 map of
Egypt, a revised edition of which is in preparation. They were then
reduced to the million scale, and afterwards still further reduced, with
additions from my own recent observations around Owenat, Gebel Kissu,
and Merga, from Hassanein Bey’s records of his journey between Jalo and
El Fasher, from the altitude-data obtained by the Anglo-French Boundary
Commission in the Sudan,[2] and from the maps of Prince Kemal el Din’s
expedition of 1926 to Owenat and Sarra Well,[3] to form the map
following p. 96. The attempt has, I think, fully justified itself, and
more than fulfilled the hopes that were entertained concerning it.
Probably the feature that will first strike the attention on glancing at
the contoured map is the series of depressions below sea-level
stretching westward from near Cairo to Jarabub. The following table
gives a list of the principal of these depressions, with the approximate
extent of the areas lying below sea-level, and also (where known) the
depths below sea-level of their deepest parts. Where the lowest part is
covered by a lake, the depth shown is that of the water-surface, not
that of the lake-bottom:
_Approximate area _Approximate
_Depression._ below sea-level depth of deepest
in sq. kiloms._ part below sea
in metres._
Wadi Natrun 220 23
Faiyum 700 45
Wadi Rayan 280 43
Qattara (including Moghara, 18,000 134
Qara, and Tebaghbagh)
Sittra (including Timata) 300 33
Bahrein (including Nuemisa) 150 15 (?)
Watiya 70 15
Areg 60 25
Siwa (including Maasir and 800 17
Maraqi)
Girba 20 (?)
Kheiba 40 (?)
Shiyata 20 (?)
Melfa (including Shebat, 260 18 (?)
Exabia, and Gagub)
As will be seen from the figures in the table, by far the largest and
deepest depression in this series—seven times as large, in fact, as all
the others put together—is that of Qattara, which has not been shown on
any previous map, and of which the existence was proved only last year.
That so deep a depression should exist comparatively near to the
Mediterranean and to Cairo, and have remained undiscovered until now, is
striking evidence of how much has still to be learned concerning the
configuration of the desert surface. The manner in which the discovery
was made is perhaps worth placing on record. Hearing early in 1917 that
a military patrol was about to operate in the Qattara region, I lent a
small aneroid to the officer in charge of the patrol, and asked him to
take readings with it at various points along his route, with the object
of getting some idea of the altitudes in that region. It was, of course,
certain that low-lying country existed along the foot of the Qattara
escarpment, but no idea was entertained that any great part of it might
be below the level of the sea. The officer brought back aneroid-readings
which seemed to indicate that the spring at the foot of the scarp at
Qattara was about 60 metres below sea-level; but the aneroid itself was
unfortunately lost during the return journey. The result was too
surprising for it to be accepted without evidence that the readings of
the instrument had not been vitiated by accidental rough treatment, and
consequently I refrained at the time from placing the levels found on
the maps. But I resolved to confirm them or otherwise on the first
opportunity.
That opportunity came last year, when it was possible to send a survey-
party to triangulate westwards from the Wadi Natrun so as to cross the
place where the aneroid readings had been taken. The work was entrusted
to Mr. G. F. Walpole, of the Survey of Egypt, who had already
distinguished himself by successfully carrying out a difficult
triangulation from the Nile across some 500 kilometres of the Libyan
Desert _viâ_ Baharia to Siwa. The result of Mr. Walpole’s work was not
only to confirm the substantial accuracy of the previous estimation of
the level of the Qattara spring, but to bring to light the existence of
a vast hollow, thousands of square kilometres of whose floor lie at even
lower levels, and which at one place descends to a depth of no less than
134 metres below the level of the sea.[4] This last-mentioned spot, some
100 kilometres south-south-west of the Qattara spring, is probably the
lowest-lying point on the land-surface of the African continent.
A scarcely less remarkable feature is the series of depressions
comprising the oases of Kharga, Dakhla, Abu Mungar, and Farafra; while
Baharia occupies a more isolated position about midway between Farafra
and the Faiyum. In contrast with those of the northern series, these
southern depressions do not penetrate to sea-level. The lowest points in
Kharga Oasis are probably at or just below the level of the sea, but the
general level of the Kharga floor is about 70 metres above that datum.
The lowest point of Farafra is somewhat higher than the general level of
Kharga; those of Dakhla and Abu Mungar are higher still, while in
Baharia, though it lies farther to the north, the floor-level averages
some 130 metres above sea.
A noteworthy thing about the larger depressions is that their northern
boundaries are formed by steep escarpments, while to the south the slope
is more gradual. This is the case, for instance, with the Faiyum,
Qattara, Siwa, Kharga, Dakhla, Abu Mungar, and Farafra. Baharia is
unique in being entirely surrounded by escarpments, as also in
containing a large number of hills approximating in height to that of
the bounding scarps. In the majority of cases the greater steepness of
the northern boundaries can be correlated with geological structure; the
northern walls of the Qattara and Siwa depressions, for instance, mark
the southern limit of certain Miocene strata, while those of Kharga and
Dakhla coincide with the southern limit of the Eocene limestones.
To the south-west of Dakhla Oasis there stretches a broad tract of
rising ground, with the Gilf Kebir and the peaks of Arkenu, Owenat, and
Kissu as conspicuous features. On either side of this tract the general
level falls, on the one hand towards Kufra and Cyrenaica, and on the
other hand towards the Nile. The Gilf Kebir is a great broken plateau of
sandstone, rising very nearly to the 1000-metre contour and stretching
in a direction a little west of south for over 100 kilometres. I saw the
south end of this plateau from several points on the way from Terfawi to
Owenat when travelling with Prince Kemal el Din in 1925, and fixed its
position by compass-bearings. As seen from the south, the plateau
appeared to be merely a flat-topped hill; its true form and extent were
discovered by the Prince in the following year, when he passed along its
eastern foot on his way from Pottery Hill to Owenat. The highest peak of
Arkenu, in about lat. 22° 17′, long. 24° 46′, I estimated to rise to
approximately 1410 metres above sea. The altitude of Gebel Owenat far
transcends Hassanein Bey’s previous estimate of 1100 metres; a
trigonometric measurement which I made in 1925 gave its summit as 1907
metres above sea. The general level of the ground at the foot of Owenat
I found to average about 600 metres, thus agreeing with Hassanein Bey’s
figures. Gebel Kissu, though lower than Gebel Owenat, is considerably
higher than Gebel Arkenu, and being an isolated mountain with a single
well-marked peak, it forms a very conspicuous landmark; my trigonometric
determination gave 1726 metres for the altitude of the peak above sea.
To the south-west of Kissu there are a few widely scattered isolated
hills, but the country in general forms a rolling plain of sand
extending to the north-east corner of French Equatorial Africa with a
nearly uniform level of about 700 metres. Beyond the French boundary the
ground rises towards the Erdi Hills, which attain over 1000 metres.
From the corner of French Equatorial Africa to Merga, a distance of 250
kilometres, the country consists of alternations of sand-plains and flat
stony tracts, with a very gradual fall to a level of about 560 metres at
the edge of the Merga depression.
Between Merga and Owenat the country consists of sandy plains
alternating with broken stony ground and occasional hills. The general
level rises from about 570 metres on the northern side of the Merga
depression to nearly 800 metres on the parallel of 20° 30′, then
gradually falls to about 600 metres at the foot of Gebel Owenat.
1. _Did the Nile, or a Branch of it, ever flow through the Libyan Desert
to the Mediterranean?_
The idea that a “dry river” exists in the Libyan Desert is a very old
one, and seems to have had its origin in the fact that barren
depressions in that region are sometimes called by the local Arabs _Bahr
bela ma_, that is, “sea without water.” But the Arabic word _bahr_,
which properly means a sea or a lake, as in Bahr Lot (the Dead Sea), is
also applied in Egypt and the Sudan to a river, as in Bahr el Nil (the
Nile) and Bahr el Azrak (the Blue Nile).[5]
The earliest depression to be discovered bearing the name of _Bahr bela
ma_ was the one now known as the Wadi Faregh (the “empty wadi”),
situated immediately south of the Wadi Natrun. Father Sicard, who
visited the place in 1712, correctly interpreted the Arabic name to mean
literally “sea without water,” and having discovered fossil trees there,
which he imagined to be the petrified remains of masts of ships, he
inferred that a narrow arm of the sea formerly extended from the
Mediterranean into the locality.[6] At a later date Sicard’s view that
the petrified trees were the masts of ships was recognized to be a
mistaken one; but a greater error was committed by changing the
translation of _Bahr bela ma_ from “sea without water” into “river
without water,” as was done by D’Anville in his ‘Mémoires sur l’Egypte,’
published at Paris in 1766. D’Anville was a great authority in his day,
and his translation was adopted by subsequent writers for over a hundred
years. General Andreossi, who commanded Napoleon’s artillery in the
expedition of 1798 and 1799, made a map of the “Dry River” and the
“Valley of the Natron Lakes,” in which both depressions are shown open
at both ends, instead of being closed in as they really are. Andreossi
considered that the “Bahr bela ma” was the dry bed of an ancient branch
of the Nile, which left the present Nile Valley somewhere in Middle
Egypt and entered the sea to the west of Alexandria; he also thought
that the ancient Lake Moeris was probably formed by the damming of the
“dry river” near its supposed offtake from the Nile.[7] Later on, after
Cailliaud and other early nineteenth-century travellers had discovered
that other depressions existed in widely separated localities and bore
the same Arabic name of _Bahr bela ma_, it seems to have been hastily
assumed by the cartographers of the time that all the depressions
bearing the same name were parts of a single dried-up river-channel.
Thus, on a large map of Egypt compiled by Colonel Lapin and published at
Paris in 1856, the “Bahar Belah-mah ou Fleuve sans eau” is stated to
come from the Congo and is depicted as coursing through the Oasis of
Dakhla, thence passing to the east of Baharia Oasis and on to the
Mediterranean. On another large map of Egypt, compiled by Muzzi Bey, the
then Director-General of Egyptian Posts, and published in Florence in
1876, a continuous valley is boldly shown leading from the mountains of
Darfur to the Mediterranean, while cross-valleys, labelled “Old Bed of
the Nile,” are depicted as connecting the main valley with that of the
Nile at Korosko and Dongola.
The Rohlfs expedition of 1874 proved that there was no such channel
running northwards through Dakhla Oasis; that the “Bahr bela ma” which
had been crossed by Cailliaud between Siwa and Baharia was merely a
closed-in local depression, and that the continuous empty river-beds
which were shown on the maps of that period had no real existence.[8]
But though the continuous “Bahr bela ma” channel of the old
cartographers has been shown to be purely imaginary, and has in
consequence disappeared from our modern maps, the belief that an old dry
river-bed _may_ exist _somewhere_ in the Libyan Desert has apparently
persisted in many minds even to our own day. Only a few years ago I was
called upon by the Government to discuss a suggestion, made in all
seriousness by a person of considerable eminence, that the Nile, or a
branch of it, must at one time have followed a course from somewhere
near Dongola through some of the Egyptian oases to the Mediterranean;
and that by tracing out the old channel, and deepening it artificially
where necessary, a part of the river might be taken off along this path
and its water utilized to irrigate the desert country on either side of
it. To the few scientific travellers who have journeyed extensively in
the Libyan Desert, it will doubtless appear incredible that such a
suggestion as this could be seriously made. But old traditions
concerning the geography of little-known regions die hard, and this
particular one has probably been fostered by the circumstance that on
most small-scale maps the oasis-depressions have been shown without any
precise indication of the altitude of the intervening ground, as well as
by the speculations of geologists as to the existence of a river in the
region in past geological ages. Blanckenhorn, for instance, published in
1902 a series of small-scale maps depicting the course of a hypothetical
river, the “Libyan Ur-Nile,” running northwards through the desert in
the Eocene and Oligocene periods, and ceasing to exist in the
Pliocene.[9]
That a river flowed during late Eocene or early Oligocene times in a
north-easterly direction through what is now the Libyan Desert, and
entered a sea near what is now the Faiyum, can scarcely be questioned;
for it is only by the existence of such a river that we can account for
the assemblage of fossil trees and curious remains of Eocene animals
(including the ancestors of the elephant) discovered by Mr. Beadnell
north of the Birket el Qarun and described by the late Dr. Andrews.[10]
As to the size, length, and exact position of this river of the
geological past there is very great doubt; but neither its situation nor
even its existence can be considered material to our present inquiry. No
one conversant with the known facts bearing on the general geological
history of Egypt during late Tertiary and Quaternary times could make
the mistake of thinking that the path of an Eocene river might be
traceable in the present desert relief. The evidences of such path must
be purely of a geological nature. In his endeavours to suggest what
might possibly have been the course of the river on whose banks the
Eocene animals lived, Mr. Beadnell found in two localities deposits
which might have been formed along the ancient drainage-line; and in
both these localities the deposits now occupy the summits of hills.
There is insufficient evidence of any kind to justify so exact a
location of the Eocene river as Blanckenhorn’s maps would suggest, and
none whatever of any relationship, either between the present surface-
configuration of the desert and Eocene or Oligocene drainage-lines, or
between those drainage-lines and the River Nile. The appellation “Ur-
Nile” of Blanckenhorn is thus a misleading one. If we are to look for
traces of a dried-up river in the surface-forms of the Libyan Desert of
to-day, it must be a river which, like the streams which eroded the
great wadis of the Eastern Desert, existed in geologically recent times,
and which became dry either through the continual deepening of the
present Nile-channel, with consequent capturing of the former drainage,
or through climatic changes causing a diminution of rainfall in the
collecting area.
A study of the contoured map seems to me to be absolutely conclusive on
the question at issue, for it indicates clearly that at no time within
the geologically recent period can there have been any such continuous
channel as has been supposed. Let us try and trace, with the aid of the
map, the most likely course along which the ancient Libyan Nile-branch,
if ever one existed, must have flowed. Starting from the Nile in the
neighbourhood of Dongola, we observe that the lowest possible track is
through Sheb towards Kharga. But Sheb can only be reached after passing
over some 350 kilometres of desert lying well above the level of the
Nile at Dongola, which is about 230 metres above sea. It is, of course,
quite conceivable that even within geologically recent times the Nile at
Dongola may have been considerably above its present level; so that we
may pass over this first difficulty as not being definitely conclusive
against the hypothesis we are pursuing. From Sheb to the south end of
the Kharga depression there is a drop of about 160 metres, so that up to
this point a channel may have been possible. But from here onwards,
difficulties come in. If we turn northwards, we soon encounter the north
wall of Kharga, rising to 300 metres above sea-level; so there is no
continuing that way. Instead, we turn westwards towards Dakhla. But from
Kharga we have to pass over ground reaching very nearly to the 200-metre
contour for some 80 kilometres to enter the Dakhla depression, where we
drop to a level of 119 metres. We have to traverse some 15 kilometres at
altitudes above 200 metres to reach Abu Mungar, where the level is 117
metres; then a further 12 kilometres lying above the 200-metre contour
to get to Farafra, where the level descends to about 90 metres. To get
out of Farafra, we have again to traverse country lying over 200 metres
above sea, whether we choose a north-westerly route _viâ_ Sittra to the
Qattara depression, or a north-westerly one _viâ_ Baharia to the Wadi
Rayan and the Faiyum. Taking the first of these alternatives as being
the shorter, we cross some 30 kilometres of ground above the 200-metre
level, and a further 120 kilometres lying between the 200- and 100-metre
contours, before reaching the Qattara depression. And once we are in the
Qattara depression (the lowest point of which is 134 metres below sea),
there is no possibility of reaching the Mediterranean except by crossing
a considerable tract where the ground-level is over 100 metres above
sea-level.
A section of the route described, with a comparison-line passing through
the present Nile-levels in corresponding latitudes, is shown in the
figure below:
[Illustration: _Profile from the Nile at Dongola through the depressions
of Kharga, Dakhla, Abu Mungar, Farafra, and Qattara to the
Mediterranean. Vertical scale 500 times the horizontal. Total length of
section about 1700 kilometres; for more than half this distance the
ground-level is seen to be above a line drawn through the levels of the
Nile in corresponding latitudes._]
Can any one believe that a course of this kind was ever that of a river?
To make a through channel would entail deep cutting for more than half
the total distance of 1700 kilometres, and would consequently involve
excavating hundreds of millions of cubic metres of rock, much of it of
considerable hardness. Moreover, it cannot be supposed, in the present
state of our knowledge of the region, that more favourable lines of
communication exist and have been overlooked. Many depressions doubtless
still remain to be discovered; but these, like those already known, will
surely prove to be merely local basins. So many cross-country journeys
have now been made in various directions through the desert, that it is
inconceivable that any continuous channel of the kind we are discussing
could possibly have escaped notice. Nor can we reasonably suppose that
recent earth-movements have produced the present severance of the
various depressions; such movements might perhaps be invoked to account
for one or two of the separating elevations, but certainly not for all.
We can therefore be absolutely certain that neither the Nile nor any
branch of it ever passed through the Libyan Desert to the sea.[11]
Even channels of tributary streams to the Nile, such as abundantly
survive in the great wadis which enter the Nile valley on its eastern
side, are markedly absent in the Libyan Desert. The contours on the map
suggest that possibly one such tributary stream may formerly have
drained the south-western faces of the great plateau in which the
depressions of Dakhla and Kharga form bays, and have entered the Nile
somewhere between Aswan and Halfa; but if such a channel ever did exist,
its actual traces have long since been obliterated by the smoothing
action of subsequent denudation, and only the great escarpment of the
plateau which it half encircled remains to tell the tale.
_Photographs by Prince Kemal el Din, Dr. Ball, and Mr. Walpole_
[Illustration: 1. _Desert exploration with the ordinary car, requiring
help even on flat ground in sandy tracts_]
[Illustration: 2. _Caterpillar cars carrying heavy loads over a sand-
dune between Bir Terfawi and Gebel Owenat_]
[Illustration: 3. _The western wall of the Qattara depression 35 km.
S.S.W. of Qara Oasis. Top of cliff about 3 m. and floor of depression
133 m. below sea-level_]
[Illustration: 4. _Ain el Hez, Baharia Oasis, irrigated by free flow
from well_]
[Illustration: 5. _Overlooking the Baharia depression from the pass at
its southern end: cars bound for Farafra_]
[Illustration: 6. _Northern end of Farafra depression: sandy ravine
cutting back into chalk plateau_]
2. _Origin of the Depressions._
It is a marked characteristic of almost all true deserts, that they are
regions of internal drainage; and the Libyan Desert is no exception to
the rule. There are a few gullies draining from the plateau to the sea
along its northern edge, and a few others draining into the Nile Valley
along its eastern border. But none of these external drainage-lines
extends for more than an insignificant distance backward into the
interior of the country. There is, in fact, in the Libyan Desert
(especially in its north-eastern portion) a marked absence of any
distinct drainage-lines at all; in their place we find extensive stony
plains, which either slope gradually down towards the various inland
depressions, or terminate in an abrupt fall where the depressions are
cut back into them. Even in the Pluvial epoch, when Europe was glaciated
and the deep wadis of the Eastern Desert on either side of the Nile were
being cut out by great streams, there cannot have been any external
drainage from a large part of the Libyan Desert; for if there had been,
we should find the Libyan plateau far more intensely dissected than is
actually the case. The rainfall in this region, even during the Pluvial
period, must have been far smaller than in the neighbouring tracts of
the Eastern Desert and Sinai; such rainfall as did occur was drained
into the series of depressions already described, and there either
formed lakes or was evaporated. But the action of inflowing drainage
waters, carrying sand and mud in suspension and salts in solution, is
not to _deepen_ the basins into which they flow, but rather to _fill
them up_. Hence we must look to some other cause for the formation of
the basins themselves. A possible cause is to be found in earth-
movements—either a local down-folding of the crust, or an upraising by
faulting of the surrounding tracts. It is not unlikely that such earth-
movements have to some extent conditioned the formation of the
depressions; but that the depressions are not simply faulted-down areas
or subsidences is abundantly clear from an examination of the bounding
scarps and the floors of the larger oases. Baharia is wholly, and Kharga
and Dakhla are partially, bounded by high escarpments, in many places
hundreds of metres high, and the floors of these oases are composed of
the same rocks as are exposed at the base of the bounding escarpments.
It is, in fact, obvious that these great hollows are natural
_excavations_, not subsidences. What was the excavating agency?
To the south-west of the limestone plateau which lies west of the Nile
and extends northwards from lat. 23° 30′ there is fairly open sandstone
country, and the oases of Dakhla and Kharga occupy great bays in the
escarpment which marks the south-west termination of the plateau. It
seems reasonable to suppose that the Eocene and Cretaceous seas had
their southern shores somewhere about the latitude of Wadi Halfa, for
the limestones do not extend far beyond the tropic; and to the south the
hard limestones of the plateau probably passed gradually into softer
shallow-water deposits, which would lend themselves easily to
degradation and to removal by the agency of streams whose traces have
long since disappeared. We can thus possibly invoke water-action to
account for the primitive formation of the “bays” in which Kharga and
Dakhla are situated. But we cannot in this way explain either the
further deepening of these oases, which converted them into basins, or
the excavation of the more northerly depressions such as Baharia and
Qattara.
The problem as to how the excavation of the northern depressions, and
the deepening of the southern ones, were accomplished is not altogether
an easy one. There is, of course, no doubt that wind has been the main
excavating agency. All the depressions occur in areas where soft rocks
are overlain by hard ones, and once the hard overlying rock was removed
at any place, the action of the wind on the softer beds would soon
excavate a hollow. The difficulty is only as to how the hard overlying
rock was first removed. To some who have studied the question, a
sufficient explanation is to be found in the general degradation,
combined with the tectonic disturbances—folds and faults—which are known
to have affected some of the oasis-areas, as, for instance, that of
Baharia. Given an anticlinal fold, a general lowering of the surface by
wind-degradation would eventually result in the exposure of the soft
underlying beds at the highest point of the fold, and subsequently
continued wind-erosion would proceed most rapidly on these soft
underlying rocks. Others, while admitting this explanation as partly
accounting for the formation of the depressions, consider that other
factors must have assisted to cause penetration of the hard overlying
beds, especially in the areas where folding is not very marked.
Of such other factors, I can only suggest what appears to me to be a
possible one. About twenty years ago I was mapping the Mediterranean
coastal area in the neighbourhood of Mersa Matruh, some 160 miles to the
west of Alexandria. In this area, which has about 6 inches of rainfall
in an average year, there are numerous closed-in basins a few miles long
and a few hundred yards in width, descending to a depth of 10 metres or
more in the rather soft limestone which forms the principal rock of the
locality. These basins, though called wadis on the maps, are more
usually known as _’ebs_ or “bosoms” by the local Arabs. I was at first
much puzzled to account for them; but eventually I arrived at what I
believe to be the true explanation of their formation. The basins, of
course, collect the surrounding rainfall in the winter, and the drainage
carries with it into the hollows a certain amount of fine loam and sand
from the disintegration of the surrounding surfaces. After the rains,
the bottoms of the basins become covered with vegetation, while the
surrounding rocky surfaces are left bare. In the hot dry summer,
however, the vegetation perishes, and much of the loam is removed by the
wind. We have only to assume (what seems indeed likely) that the
rootlets of the vegetation disintegrate the limestone floors of the
hollows to form fresh soil, and that more of this is removed each year
by the wind than is brought into the hollows by the drainage, to find an
adequate cause for a slow but progressive deepening of the basins. The
primitive hollow may well be a very slight depression of the surface,
such as might arise from greater local softness of the rock and
consequent more rapid weathering. Once vegetation has commenced to grow
in a slight hollow of this kind, the action would lead to a slow yet
steadily progressive deepening year by year, and sand-erosion would tend
towards an elongation of the depression in the direction of the
prevailing wind. Can such a process as this, continued through vast ages
during which the climate was somewhat less dry than at present, have
been active in the formation of Baharia and similar depressions?
3. _The Possibility of Utilization of Depressions in the Libyan Desert
for Irrigation, Drainage, or Hydraulic Power._
The depression known as the Wadi Rayan, discovered by Mr. Cope
Whitehouse about 1883, situated some 40 kilometres west of the Nile
Valley and about 100 kilometres south of the latitude of Cairo, has been
so frequently described and considered in regard to its utilization as a
reservoir or as a flood-protection for Lower Egypt, that I need say
little about it beyond a reference to the principal published works
dealing with it.[12] There is no other depression in the Libyan Desert
which has received so much attention from the economic standpoint. That
the Wadi Rayan has not already been utilized as a reservoir has depended
on a variety of considerations, of which one of the principal has been
the doubts always entertained as to whether there might not be an
underground leakage from it, resulting not only in a loss of part of the
stored water, but also in damage to the adjacent flourishing province of
the Faiyum. These doubts will not be lessened by the converging lines of
evidence which I shall bring forward in dealing with the artesian water-
supplies, all leading to the conclusion that there is an underground
water-connection between the various depressions of the Egyptian Libyan
Desert. Of late years, however, it has been proposed to make use of the
Wadi Rayan, not as a reservoir, but as a sump for disposing of the
drainage water of Upper Egypt by evaporation; and to this, in
consequence of the lower level at which the water would stand in the
depression, there is far less objection from the point of view of
possible leakage or damage to the Faiyum.
The Wadi Natrun, which, though considerably less deep, is not greatly
less in extent than the Wadi Rayan, has received some consideration as a
possible sump for disposing of some of the drainage water of the Behera
Province of the Delta. So long ago as 1895 the Public Works Department
studied the possibility of draining Lake Mariut into the Wadi Natrun,
and thus doing away with the increasing cost of keeping down the level
of the lake by pumping water from it into the sea.[13] The Wadi Natrun
has an area, even at the low contour of 10 metres below sea-level, of
nearly 100 square kilometres, and the average rate of evaporation from
an open water-surface within it could not well be much less than 3 mm.
per day; the wadi could therefore dispose annually of at least 100
million cubic metres of drainage water by evaporation alone, and
probably of a considerable additional quantity by seepage. But the high
cost of cutting a drainage channel from Lake Mariut to the wadi, and the
heavy maintenance charges which would be incurred in keeping the drain
open, were formidable objections to the scheme. From a reconnaissance
carried out in 1896, Mr. Verschoyle found that the length of the
proposed drain would have to be almost 80 kilometres, and for the
greater part of the way it would have to be a cutting 20 metres in depth
through desert and ridges of drifting sand; he remarked that if it were
an easy matter to make the connection, it would be no easy matter to
maintain it; and he concluded that the scheme was an impracticable
one.[14] A more recent proposal has been to construct a partly tunnelled
channel instead of an open drain; but this has likewise been found to be
impracticable, as involving too heavy an outlay for the benefits which
would result.[15]
[Illustration: 7. _Qasr Farafra, seen from the north over plain of
powdery chalk forming floor of depression_]
[Illustration: 8. _Escarpment and “Hattia” of Abu Mungar from the
south_]
[Illustration: 9. _The “Hattia” of Abu Mungar from the east: well on
earth-mound on right_]
[Illustration: 10. _The principal well at Qasr Farafra_]
[Illustration: 11. _A street in Qasr Farafra, with irrigation canal_]
When the great extent and depth of the Qattara depression were
established by Mr. Walpole’s explorations of last year, hopes were
immediately entertained that the depression might be made to serve some
useful economic purpose. Any idea of its being of use as a Nile
reservoir was of course at once cut out, not only by reason of its
position and distance from the Nile, but also by its immense size; for
even if we could turn the whole Nile flood into it, some twenty years or
more would be occupied in filling it to a sufficiently high level,[16]
and the loss by evaporation from so large an area would be enormous. As
a receptacle for drainage water from the Delta it appeared equally
impossible of consideration, because of the great length and depth of
the channels which would have to be cut to reach it. A more reasonable
prospect seemed to be that of admitting sea-water from the Mediterranean
into it by means of a navigable canal from the Arabs Gulf; this would
only have to be about 56 kilometres in length to reach the nearest point
of the depression; and once the canal was made and the depression
filled, ships might sail almost to Siwa. Other advantages which might
accrue from the formation of this inland sea were an increased humidity
of the climate along the Mediterranean Littoral of Egypt, leading to
increased rain-crops in that region, the establishment of a valuable
fishery, and a possible increase in the water-supply of the oases by the
causing of a slight rise in the static water-level there. Another idea,
which rapidly followed the first one, was to utilize the depression as a
source of electrical power for driving pumps by which the drainage of
the northern parts of the Nile Delta might be improved. At first sight
this latter prospect looked a very attractive one. The salt-marsh which
covers much of the floor of the depression appeared suggestive of a
former sea-connection, and if we could trace out this old connection,
the cutting of a canal along it might not, it was thought, be a very
expensive matter. Moreover, it was apparent that evaporation from an
inland sea or lake of so large an area would keep pace with quite a
large influx from the Mediterranean, so that if the influx were
restricted to such a quantity as would permit of the lake-surface being
maintained at a level considerably below that of the Mediterranean,
power could be generated continuously. Suppose, for instance, that we
maintained a permanent water-level in the lake of 50 metres below the
sea; the estimated area of the lake at this level being 9000 square
kilometres and the mean evaporation assumed to be 4 millimetres a day,
an influx of no less than 36 million cubic metres of water per day could
be passed into the lake without altering its level, and this with a fall
of 50 metres would suffice theoretically for the continuous generation
of over 270,000 horse-power. Of course the lake would gradually get more
and more saline, but the power might continue to be maintained for very
many years before the lake became as rich in salt as the Dead Sea.
But alas! an investigation of the northern borders of the depression
showed that the hoped-for traces of a former connection with the
Mediterranean Sea do not exist. The depression is entirely shut in from
the north, either by great cliffs, or by ground lying so high that the
cutting of a canal to the sea is utterly impracticable. Of the 56½
kilometres from the sea-level contour of the depression to the coast
along a line running 12° east of north from Moghara Lake (which is the
easiest line hitherto found for the cutting of a canal), only 16
kilometres are at less than 50 metres above sea; 31 kilometres lie at
altitudes between 50 and 100 metres; and 9½ kilometres are above the
100-metre level. An open channel being thus put out of the question, it
was next natural to inquire whether a tunnel, or a channel partly open
and partly tunnelled, could be excavated to serve for the conveyance of
sea-water into the depression. But even this, though perhaps not
impracticable, would be a very costly undertaking. The conveyance of the
requisite quantity of water, even at a relatively high velocity (which
of course implies a considerable slope), would necessitate a tunnel or
tunnels of very large dimensions. To convey 36 million cubic metres a
day at a velocity of 5 kilometres per hour would require a sectional
area of 300 square metres—_i.e._ if two tunnels were made, each would
have to be 14 metres in diameter. The cost of boring and lining such
tunnels would certainly be very great, and it is doubtful whether the
value of the power generated could justify the capital expenditure
involved in the excavation and other works which would be necessary.
Until more is known of the nature of the strata through which the
aqueduct would have to pass, and as to the length of tunnelling and
amount of open cutting which would be required, it is impossible to form
even an approximate estimate of the expense which the undertaking would
involve. All that can be said at present is that the utilization of the
Qattara Depression offers a possible means of obtaining power on a large
scale for the drainage of the low-lying lands of the Delta, with at the
same time a prospect of improving in some measure the agricultural
resources of the Mediterranean Littoral; much further investigation will
have to be accomplished before any judgment can be formed as to whether
such a project would be an economically sound one.
In formulating any scheme for improving the drainage of the Delta, it is
of course important to consider, not only the manner in which the
drainage water could be ultimately disposed of, but also the
modifications which would have to be made in the existing drains and
irrigation canals—modifications which would need to be carried out
without serious interruption to existing agriculture. A scheme which
otherwise appeared attractive might easily prove to be impracticable by
reason of the heavy expenses and inconveniences of the subsidiary works
which would be required to make it effective.
[Illustration: THE LIBYAN DESERT Showing Surface-Relief, Contours of
Static Underground Water-levels, Distribution of Sand-dunes, and Routes
of Principal Exploratory Expeditions in the West and South by Dr. JOHN
BALL
THE GEOGRAPHICAL JOURNAL JULY 1927]
4. _The Artesian Water Supplies._
The origin of the artesian water supplies of the Egyptian oases of
Baharia, Kharga, Dakhla, and Farafra (Siwa seems hitherto generally to
have been left out of consideration) has been a much-discussed question.
Some geologists, myself amongst them, have always regarded the water as
being derived from rainfall in the western Sudan, flowing underground in
permeable beds towards the Mediterranean. Others have held the view that
the oasis waters are merely Nile water which has penetrated more or less
laterally into the adjoining deserts. The arguments that have been urged
in support of the former view are, firstly, the high temperature of the
water in many of the oasis wells; and, secondly, that the levels of the
springs and wells are often much higher than those of the Nile in the
same latitudes. To these arguments it has been justly replied that
neither of them is conclusive; the high temperature of the outflowing
water merely testifies that it has descended to considerable depths at
some part of its underground path not very remote from the point of
outflow, but really tells us nothing as to its place of origin; and the
high level of the springs in Baharia, for instance, as compared with
that of the Nile in the same latitude, might be accounted for by the
seepage from the Nile taking place fairly high up in the river’s course.
There the question remained until 1925, when I was able to visit and
determine the positions and levels of a number of water-sources farther
in the interior than any of those on which the “Nile” argument was
based. Amongst other level-determinations, I ascertained that the Sheb
well is 228 metres above sea-level, and that Merga Lake, lying far to
the south-west (in lat. 19° 3′, long. 26° 18′), is at an altitude of no
less than 509 metres above the sea. Shortly before my tour in the Sudan,
Hassanein Bey had confirmed Rohlfs’ level of 400 metres for the Kufra
water-sources, and I had found that of Abu Mungar (north-west of Dakhla
Oasis) to be 117 metres. At all these places the water-supplies are
derived from underground sources in the same rock—namely, the Nubian
sandstone, which covers such vast areas in the Sudan and Egypt.
I had thus four well-determined natural water-levels at the corners of a
great quadrilateral whose sides averaged over 500 kilometres in length,
and embraced more than 20 square degrees of the Earth’s surface. Now
just as in solid geometry the levels of any three points determine the
inclination of an oblique plane to the horizontal, so on the Earth any
three levels will determine the inclination of a surface to the geoid
(of course assuming both geoid and surface to have the same curvature).
And on making the calculation, I found that I obtained practically the
same degree and direction of inclination for the natural water-surface,
whichever three of the four known points I utilized for the calculation.
In other words, I found that if I took, say, the levels of Kufra, Abu
Mungar, and Sheb, and deduced from them the inclination of the water-
surface to the horizontal, I could _calculate_ the level of Merga pretty
exactly. Extending the trial, I found that I could do the same with a
fairly close approximation for the other wells in the Sheb
neighbourhood, and also for wells in the oases of Dakhla and Kharga. The
conclusion seemed irresistible that all the wells considered were fed
from a continuous sheet of underground water; and it was evident that
this water did not come from the Nile, firstly because of the high level
of Merga, which is above that of the swamps of the Bahr el Ghazal and
other western feeders of the White Nile, and secondly because of the
direction of the downward slope of the underground static water-surface,
which is from the south-west, instead of from the south as it would have
been had the water been derived from the Nile in the Bahr el Ghazal
region. The true source of the water must be somewhere more or less
nearly on a line drawn south-west from Dakhla, for this is the direction
of upward slope of the underground static water-surface; and if such a
line be drawn on a map of Africa, it will be found to lead towards the
Erdi and Ennedi region, on the borders of the Chad basin. It is in the
highlands of Eastern Erdi and Ennedi, therefore, that we must look for
the source of the artesian water of the Egyptian oases. What is known of
this region from the recent explorations of Colonel Tilho lends good
support to our conclusion.[17] It is a bare and rugged sandstone
country, where, in spite of a rainfall which is by no means negligible,
permanent water-sources are scanty, and where, in consequence, there
must be a considerable absorption of moisture by the rocks; and it lies
at so high an altitude as to give sufficient “head” for the absorbed
water to percolate through the porous sandstones and thus to reach
Egypt.
Being convinced that I had at last arrived at the true origin of the
artesian water, I next began to entertain the idea of attempting to make
a map which would show the contours of the underground water-sheet, and
from which, in conjunction with the contour-map of the surface which I
had already prepared, I might be able to predict the depth of boring
required to tap the underground water at any point in the desert. But a
little consideration showed that this idea was an impracticable one, by
reason of our ignorance of the underground geological structure over the
greater part of the desert. The underground water would naturally pass
along permeable sandstone beds, often confined between impermeable clays
above and below. And although the general structure of the Libyan Desert
is doubtless one of simplicity as compared with that of other parts of
Egypt, yet we know, from observations in the oases and in the Owenat
region, that the beds are in some places folded and faulted, and that in
others they have been uplifted and entirely removed by denudation, with
the exposure of large areas of the underlying ancient crystalline rocks.
The only parts of the desert for which the boring-depths could safely be
predicted would be certain small areas within the oases, where wells
have been sunk in sufficient numbers to give us definite information as
to the local underground structure; and underground-water maps of these
small areas, though they might usefully systematize our knowledge
concerning them, would not be of any use for predictions at points
situated elsewhere in the deserts.
But while it was thus impracticable to prepare maps showing everywhere
the depth at which underground water actually exists, I conceived that
it might be quite possible to prepare maps showing _static water-
levels_; that is, the levels to which the water would anywhere rise
hydrostatically when once it _was_ tapped by borings. For the slope of
the static water-surface between known points will be largely
independent of the underground structure of the intervening country.
Apart from any physical changes which may still be going on in the
underground rocks themselves through geological agencies—changes which,
if taking place at all, must be so slow as to be negligible except in
the course of centuries—the only factors which can affect the slope of
the static water-surface, once it has been established, are variations
either in the rate of supply of water to the beds, or in the rate at
which it is removed from them. As regards variations in the rate of
supply, it is obvious that variations in the annual rainfall of the Erdi
and Ennedi region must cause very considerable variations from year to
year in the amount of water received by the underground beds. But the
resistance of friction to the flow of water through the pores of the
sandstones is so great, that the annual oscillations of pressure must be
rapidly damped out as the distance from the place of influx increases;
consequently the levels of the water in the wells of the Egyptian oases
(and even, so far as is known, that of the lake at Merga) show little or
no annual variation. And with regard to variations in the rate of
removal of water from the beds (by outflow to the Nile, or to the sea,
or into lakes wherein it evaporates, or by the exploitation of wells and
springs for irrigation purposes), these changes, though possibly in some
cases they may be progressive, and in restricted localities very
sensible, can exercise but little influence from year to year on the
general distribution of water-pressure within the underground strata. We
may therefore conclude that the gradient of the static water-surface
will everywhere have assumed practically a steady state. Unlike the
actual water bearing beds themselves, which may be much folded, the
static water-surface will in general have simple gentle slopes
everywhere in the open desert. In the oases, of course, where numbers of
wells yielding large outputs have been bored in proximity to each other,
the static water surface will be wrinkled; but over the vast bulk of the
desert the contours may be expected to be smooth curves. The
diagrammatic section below will, I think, make clear this point about
the general non-dependence of the shape of the static water-surface on
the geological structure:
[Illustration: _Diagrammatic section showing that the static water-level
is largely independent of the underground structure_]
In the diagram, FEKHG represents a water-conveying stratum, folded
throughout its course and faulted at HK, but having a general downward
slope from F to G. A and B represent points at which the water just
rises to the ground-level, either through natural fissures or in
artificial borings. The straight line drawn through A and B represents
very approximately the static level at any point between A and B, that
is, the level to which the water would rise in bores carried down to the
water-bearing bed. A boring at C, for instance, would have to go down to
E to tap the water, but once the bed was tapped the water would rise in
the bore as far as D. At the fault HK, the pressure of the water at K
will cause it to rise through the crushed rock at the fault-plane and
re-enter the porous stratum at H. If there is a considerable outflow at
B, and the fault-plane is a very narrow fissure, we may expect some drop
in the line AB over the fault, by reason of the extra frictional
absorption of head at this place. But unless the thickness and degree of
permeability of the fault-rock are markedly different from those of the
sandstone bed itself, the drop of pressure will not greatly disturb the
general slope AB. In any case it is apparent that the level of the
static water-surface at any place between A and B is capable of being
estimated with a far closer degree of approximation than is the level of
the water-sheet itself. We may therefore justifiably assume a uniform
gradient for the static level between points at which that level is
known, disregarding folds in the strata; and though we cannot entirely
allow in detail for unknown faults and variations in permeability, it
must be borne in mind that the _total_ effect of all the unknown factors
between any two known points is already automatically allowed for in our
data. It is only the variations from uniformity, due to the unknown
distribution of the faults and of the departures from the average
permeability, which can affect us; and these variations and departures
are probably but small in most of the great unexplored areas of the
south-west of Egypt, where the geological structure, from all we know of
it, appears generally to be remarkably uniform.
The first requisite for the construction of a map showing the contours
of the static water-surface was, of course, a sufficiency of well-
determined positions of points where the static water-level of the
artesian supply was fairly exactly known. Such points are the springs
and wells of the various oases, the surfaces of lakes occupying
depressions and presumably fed by underground supplies, and any places
on the Nile where the river taps artesian beds.
In regard to the wells and springs, it was obvious that only those known
to derive their supplies from artesian sources could be utilized as
giving points on the static water-surface. This consideration cut out
from the discussion the springs of Owenat and Arkenu, which are known to
be fed by local rainfall, and also the small water-sources of Kurkur,
Dungul, Nakheil, and Ain Amur, which occur in situations where
percolation from occasional local rainfall seems to be the only possible
source of supply. And for reasons which will appear presently, none of
the wells and springs situated to the north of the Siwa-Qattara-Faiyum
chain of depressions could be considered as entering into the problem.
With these exceptions, every water-source situated within the area of
the Libyan Desert covered by the map, and whose level was known, was
utilized; but in the greater oases and in the Wadi Natrun the wells and
springs are so numerous and so close together that in these localities
it was necessary to select one or two wells as representatives of a
group. I had no hesitation in including the wells of Siwa and the Wadi
Natrun, because the temperature of some of the wells and springs of
Siwa, and the quantity of the output of water at both places, seem to me
to afford conclusive evidence of the artesian character of their
supply.[18] The wells of the little oasis of Lageita, to the east of the
Nile near Qena, were included, for although they are not in the Libyan
Desert, they most probably derive their supplies from the same
underground flow which feeds the western oases. I have included only one
well in which the water does not rise nearly to the ground-level. That
well is one which was bored by the British Army during the Great War, at
a place called B6, some 40 kilometres to the east of Baharia Oasis. The
level of the ground at this point is 112 metres above sea-level, and as
the water was stationary in the bore at 78 metres below the ground, the
static level here is 34 metres above sea.[19] The well is said to have
yielded some 800 gallons per hour without the water-level in the bore
being sensibly changed.
The level of the well at Sarra has recently been determined by Prince
Kemal el Din; but I have not included it in my data, because he informs
me that the water-level fluctuates by 20 metres or more in different
years, while the level of the artesian water of the Egyptian oases and
Merga is very nearly constant. The inference I draw from the great
fluctuations in the water-level at the Sarra well is that it is
dependent on percolation from a more or less local rainfall rather than
on the same flow which feeds the Egyptian oases.
5. _Permanence of Lakes._
In regard to lakes and salt-marshes, the permanence of those occupying
the depressions of Areg, Bahrein, Sittra, and Qattara can only, I think,
be adequately explained by regarding them as fed, at least in part, by
underground supplies coming into them from the south. The total area of
the lakes of Bahrein, Nuemisa, Sittra, and Moghara is nearly 20 square
kilometres, and that of the salt-marshes (_sabakha_) is not less than
5000 square kilometres. The depressions are situated in a region which
is nearly rainless; in Siwa the mean annual rainfall is only about a
quarter of an inch, and that in the depressions farther south, such as
Bahrein and Sittra, is doubtless even smaller. The mean daily
evaporation from the lake-surfaces cannot well be less than some 4 mm.,
which would mean a lowering of the lake-levels by evaporation of 1½
metres each year unless there was some inflow to make up for the loss.
And though the rate of evaporation from the salt-marshes, area for area,
is doubtless very much smaller than that of the lakes, the 250-fold
greater extent of the marshes makes it certain that the total quantity
of water annually evaporated from them must far exceed that from the
lakes.
It appears unlikely that the loss by evaporation in the lakes and
marshes can be entirely made up from local rainfall and by seepages from
the northern slopes. The rocks forming the surface of the great Miocene
plateau, 200 metres high, which separates the depressions from the sea,
are chiefly limestones and clays; the beds are nearly horizontal, but
such slight dips as exist are believed to be towards the sea. The
average annual rainfall on the coastal portion of the plateau is about 6
inches; but it falls off rapidly inland, till it is only about a quarter
of an inch near Siwa. The heaviest rainfall on the plateau thus occurs
along a strip parallel to the coast, where it is largely drained off
towards the sea by the gullies which indent the plateau-edge. Of that
which falls on the plateau-surface farther inland comparatively little
is absorbed, owing to the generally non-porous nature of the uppermost
rocks; after a heavy shower, water lies on the surface in shallow pools
for a few days and is soon evaporated. So impervious to water is the
limestone in this region, that the Romans excavated chambers in it to
form reservoirs, of which many hundreds still exist. Nor can we think
that much surface drainage-water from the country to the south ever
finds its way into the depressions; for there is an almost complete
absence of drainage-lines entering them. At the feet of the northern
scarps of the Qattara depression, and along the north-eastern shores of
the lakes in the Wadi Natrun, there are, indeed, small springs which
show that some of the rain falling on the plateau does actually
penetrate the rocks and escape by seepage into the depressions. But the
amount of this seepage appears to be insignificant compared with the
volume of water which must annually disappear from the lakes and marshes
by evaporation. A further consideration bearing on this point is that
whatever may have been the agency by which the depression of Siwa was
formed, that same agency almost certainly operated to produce the other
depressions of the northern chain; and it seems most unlikely that a
connection should have been opened up with the underground water-bearing
beds in Siwa, and not also in the larger and much deeper depression of
Qattara.
It would be a difficult matter to estimate the relative proportion of
the water entering the depressions by underground flow from the south,
to that contributed by local rainfall and seepage from the northern
slopes. But that is not necessary for our immediate purpose. It is
sufficient to show that there must be _some_ influx into the depressions
from the same source as that which supplies the wells of the greater
oases, to establish the existence of that underground water-connection
which is all that we need to justify us in regarding the levels of the
lakes and salt-marshes as furnishing us with points on the static water-
surface; and from the considerations mentioned above it seems to me
certain that some influx of underground water really does take place.
I have also thought it justifiable to include the Birket el Qarun in my
collection of static water-level data, because although that lake was
probably first formed by an overflow of the Nile into the Faiyum, and is
even now being fed by Nile water through the Faiyum drains at the rate
of some 350 million tons a year, there is a certain amount of evidence
suggesting that it has some underground water-connection with the
Qattara depression. That evidence, to which attention was first drawn by
Professor Schweinfurth,[20] consists in the relatively low salinity (1·3
per cent.) of the lake, notwithstanding the long period through which it
has been subject to evaporation and the fact of its having shrunk to
dimensions very much smaller than it possessed in ancient times. Unless
there has been a large underground efflux of salt water from the lake,
it appears impossible to account for its present degree of freshness. In
Professor Schweinfurth’s day, of course, the existence of the Qattara
depression was unknown, and it was puzzling to suggest where the salt
water had gone to.[21] An underground leakage from the Birket el Qarun
into the Qattara depression is quite conceivable, for although the two
places are separated by some 200 kilometres, there is a very
considerable fall between them. Thus the salt in the marshes of the
Qattara depression may possibly have come in part from the Birket el
Qarun. The present rate of discharge of the Faiyum drains into the lake
is, however, just sufficient to make up for an average daily evaporation
from the lake-surface of a little over 4 mm., which is about the rate we
might expect; and although the level of the lake-surface has fallen some
5 metres since observations of it were first made in 1886, it is now
nearly stationary; hence it does not appear likely that there is much
underground leakage at present. If the former leakage from the lake took
place by lateral flow into porous strata near its surface, of course the
leakage may have been arrested by the lowering of the lake-level
uncovering the porous beds into which it took place; but I think a more
likely explanation is that the leakage occurred at or near the bed of
the lake, and has gradually been reduced by the continued deposition of
Nile mud on the lake-bottom, and by the diminution of head due to the
fall in the water-level.
In regard to the tapping of the artesian waters by the Nile, there is
only one locality in which this is known to take place; but the quantity
of underground water which is there withdrawn by the river is probably
very considerable. When I was surveying the Nile Valley between Aswan
and Korosko in December 1898, I observed that in the vicinity of the
temple of Dakka (about 105 kilometres south of Aswan) the lands on the
west bank of the river were being irrigated with warm water, drawn by
“sakias” (water-raising machines) from pits sunk in the alluvial flat
which extends between the river and the edge of the sandstone desert.
The length of the tract over which the warm water was being withdrawn
for irrigation was found to be about 16 kilometres, stretching from 2
kilometres north of Dakka temple southwards to the temple of Maharraga;
and the width of the alluvial tract at Dakka, where it is widest, was
about 1300 metres. Some of the water-pits were more than a kilometre
from the river. Levelling from the Nile (the surface of which was then
about 99 metres above sea) across the cultivation to one of the sakia-
pits 750 metres west of the river, I found the level of the ground at
the sakia-pit to be 7·9 metres above that of the Nile, and the water-
surface in the pit to be 8·4 metres below the ground-level; there was
1·2 metres depth of water in the pit. The temperature of the water in
the pit I found to be 83° F., while that of the Nile was 60° F. and that
of the air was 67° F. The headman of Dakka told me that the exploitation
of this warm underground water had begun about 1887; they dig out the
sandy mud, and then see the water oozing rapidly into the pit out of the
sandstone below. On crossing to the east bank of the river, I found that
there also the warm water was being similarly raised for irrigation,
though to a smaller extent, because on that side the sandstone desert
approaches more closely to the river and there is much less cultivable
land. The exploitation of the water on the east side of the Nile
extended only over a distance of about 5 kilometres along the bank, with
a maximum width of alluvial plain of 600 metres, just at the place where
the great Wadi Alagi debouches into the Nile Valley. As the sandstone
bed from which the warm water issues is less than 2 metres below the
level of the water-surface of the Nile, and the water occurs on both
sides of the river, it is certain that the water-bearing bed is cut
through by the Nile channel itself; the seepage into the river along the
stretch of 16 kilometres must therefore be very considerable. It seems
evident that the water is not derived from the bed of the Wadi Alagi,
great drainage-channel though that wadi is; for we could not then
account for the temperature of the water, nor for its appearing to a
larger extent on the west bank than on the east, with the river in
between. Moreover, the water appeared to be much more free from salts
than we should expect it to be if it were merely drainage from the Wadi
Alagi. It strongly resembles, in fact, both in temperature and
character, the artesian water of the greater oases, and there can hardly
be the smallest doubt that at Dakka the Nile is not only continually
abstracting artesian water from the same underground water-sheet that
feeds the oases, but is abstracting it in far larger quantities than
those yielded by all the oasis wells and springs put together.[22] It is
certainly remarkable that the place where considerable supplies of warm
underground water enter the Nile should coincide with the embouchure of
what is perhaps the greatest drainage channel of the Eastern Desert of
Egypt; but I think it is likely that the explanation of the coincidence
may be a tectonic one; the water-bearing beds may have been brought up
by a local fold in the strata, and the same fold may in some way have
conditioned the formation of the primitive drainage-line which was
ultimately to become the Wadi Alagi.
Having now indicated briefly the grounds for their acceptance, I give
below a table showing the various points which I have adopted as
furnishing data for constructing the contours of the static water-
surface underlying the Libyan Desert, together with the altitudes of the
points above or below sea, and the sources of these level-data. The
levels are doubtless in some cases slightly inaccurate; but a few metres
of error are immaterial to the object in view, and it is believed that
even those levels which rest on barometric determinations are
sufficiently accurate for our purpose.
LIST OF ADOPTED POINTS ON THE STATIC WATER-SURFACE
_Place._ _Level _Determined by._
(metres)._
Wadi Natrun, surface of − 23 Ball, Trigonometric
lakes levelling, 1914.
Birket el Qarun, surface of − 45 Survey of Egypt, 1926.
lake Based on spirit levelling
from Alexandria.
Moghara, surface of lake − 23 Walpole, Trigonometric
levelling, 1924.
Qattara Depression, various − 80[23] „ „ „
points on salt-marsh, the
lowest being
Sittra, surface of lake − 16 „ „ „
Areg, surface of lake − 25 „ „ „
Siwa − 17 „ „ „
Jaghbub + 32 Hassanein, Barometric
observations, 1923.
Jalo + 61 „ „ „
Bir Butaffal + 98 „ „ „
El Harrash + 310 „ „ „
Awadel (Kufra Oasis) + 434 „ „ „
Ezeila (Kufra Oasis) + 389 „ „ „
Bawitti (Baharia Oasis) + 129 Ball, 1917, and Walpole,
1924. Trigonometric
levelling.
El Hez (Baharia Oasis) + 134 „ „ „
B6 Well (water surface in) + 34 Walpole, Trigonometric
levelling, 1924, and
military records of depth,
1916.
Lageita (Eastern Desert) + 121 Murray, Trigonometric
levelling, 1921.
Farafra + 90 Ball, Barometric
observations, 1924.
Abu Mungar + 117 „ „ „
Mut (Dakhla Oasis) + 119 „ „ „
Kharga (average of numerous + 70 Beadnell, Spirit levelling,
wells) 1909.
Ain Ismail (Kharga Oasis) + 67 Ball, Barometric
observations, 1925.
Bir Murr + 156 „ „ „
Bir Abu Hussein + 182 „ „ „
Bir Kassaba + 176 „ „ „
Sheb Well + 228 „ „ „
Safsaf + 230 „ „ „
Bir Terfawi + 244 „ „ „
Merga, surface of lake + 509 „ „ „
Dakka, water-surface in + 99 Ball, Spirit levelling from
wells the Nile, 1898.
To prepare a map showing the contours of the static water-surface, I
took a graticuled sheet and plotted the above-scheduled points on it in
their ascertained geographical positions, affixing the adopted level to
each. To get points on the various contours at vertical intervals of 100
metres, I joined each pair of points on the map by a pencil line, and
then, by interpolation from the terminal levels, found the points on
this line where the various contours crossed it, on the assumption of a
uniform gradient between the terminal points. Many of the lines thus
drawn of course crossed each other, so that interpolation of the static
level at the point of their intersection gave two values for the same
place. But their agreement was wonderfully close, considering the
fewness and the scattered nature of the datum-points, and this went a
long way to encourage me in the belief that the hypothesis on which I
had been working, namely, that of an underground water-connection
between all the points included in my list, was correct. I found that
the contours of the static water-surface could be approximately
represented by a series of smooth curves, as shown (on a reduced scale)
in the outline map below.
Apart from the general smoothness of the curves, especially in the
south-west, where it may in part be due to the scantiness of control-
points, the most striking thing on this outline map is the north-
eastward projection of the 100-metre static contour, where it runs out
so as to include Baharia Oasis. The reasons for this projection are
obviously the efflux of water, on the one hand north-westwards into the
great Qattara depression, and on the other hand into the Nile at Dakka.
The indentation of the 400-metre contour near Kufra is likewise
explained by the withdrawal of water from the wells of that oasis. The
general parallelism of the curves in the south-western part of the map,
showing a gradual rise in a south-westerly direction towards the Erdi
and Ennedi country (which, as I have already stated, is the most
probable source of the underground water) is strikingly apparent. I have
not been able to extend the contours far to the east and west of Merga,
for lack of control-points. It is much to be hoped that some future
traveller will determine the water-levels at Selima and Lagia, which
would enable the static contours to be extended into the region between
Merga and Dongola; provided, of course, that an examination of the
water-sources at these places proves their supplies to be artesian.
The most effective way of testing any working hypothesis in natural
science being the prediction of hitherto unobserved facts, I venture to
forecast that if, as is most likely, the water-sources of Selima and
Lagia are artesian, their levels when eventually determined will not be
found to differ very much from 270 and 390 metres above sea
respectively. These are the approximate levels deduced by prolonging the
static water-contours of my maps into the localities of these wells,
assuming the contours to continue as smooth curves.
[Illustration: _Outline map of the Libyan Desert, showing the points
where the static water-levels are known, and the deduced contours of the
underground static water-surface, on the hypothesis of a continuous
hydraulic connection between the points_]
Another interesting prognostication which I think may fairly be deduced
from the map is that if ever the well at Sarra is considerably deepened,
a much more abundant water-supply will probably be obtainable. The
ground-level at Sarra, according to observations made by Prince Kemal el
Din, is 461 metres above sea, and the water-level in the well varies in
different years from about 390 to about 410 metres above sea. But an
examination of the static contours of the map shows that the static
level of the true artesian water in the neighbourhood of the well is
probably somewhere about 500 metres above sea, though an exact
estimation of the static level at that spot is not possible because of
the lack of data farther west. As already remarked on p. 110, I think
the present supply at Sarra is derived from more or less local rainfall,
conveyed by higher-lying permeable strata than those which convey the
main artesian supplies of Kufra and the Egyptian oases; by deepening the
well considerably, lower-lying beds might be reached so as to tap the
main supply, and the water might even be expected to overflow at the
surface.
Interesting as are the static water-level contours in themselves, they
become vastly more so when superposed on the ground-contours, as is done
in the larger map (_G.J._, July, following p. 96). From the two sets of
contours on that map we can estimate at any point the approximate depth
of the static water-level below the ground; and this information affords
new light on some of the most interesting, but hitherto the most
difficult, of the problems connected with the Libyan Desert.
[Illustration: _Pottery Hill, possible site of “Zerzura,” from the
south-west_]
[Illustration: _Bir Kassaba, a watering-place on the Darb el Arba’in_]
[Illustration: _The waterless stretch of the Darb el Arba’in between Bir
Murr and Kharga_]
[Illustration: _Bir Sheb, a well on the Darb el Ar ba’in_]
6. _Can the Present Water-supplies of the Mediterranean Littoral be
supplemented by Artesian Borings?_
The present water-supplies of the Egyptian portion of the Mediterranean
littoral, derived mainly from shallow wells dependent on the local
rainfall, are neither very abundant nor of very good quality. At one or
two of the most important settlements along the coast, such as Matruh
and Sollum, attempts have been made to improve the supplies by sinking
wells to a considerable depth in situations where it appeared likely
that the drainage from the inland plateau would be specially abundant.
But these have met with little success; the yield has been found to be
very moderate in quantity, and of poor quality owing to dissolved salts.
The question has often been raised as to whether very deep borings,
carried down right through the Tertiary strata and into the Nubian
sandstone, might result in the procuring of an artesian supply of the
same excellent water as occurs in the oases. Hitherto it has not been
possible to give a definite answer to this question, and geologists have
been reluctant to recommend deep borings, which would entail great
expense, without feeling some assurance that they would be successful.
The depth to the Nubian sandstone is unknown, but is certainly great;
and if borings were carried down into the sandstone, it was not known
whether the water would rise to anything like the ground-level. From our
new map, however, we obtain a very decisive verdict on the matter. The
Nubian sandstone, even if reached, would not be found to be charged with
artesian water under anything like the pressure that it is in the oases;
leakage into the Qattara and other depressions will have depleted the
beds of much of the water coming from the south-west, and will have
lowered the static head to such an extent, that the water left in the
sandstone will have too little pressure to rise far into the bores. Any
idea of sinking deep artesian wells along the coast to tap the Nubian
sandstone can consequently be definitely abandoned. We are driven to the
conclusion that in any attempt to improve the local water-supplies of
the littoral settlements, we can count only on local rainfall for our
primary source, and we must do our best to collect the run-off before it
has had an opportunity to absorb much salt. The Romans evidently
understood this when they excavated the large rock-cisterns on the
plateau, of which there are hundreds. We cannot do better than imitate
their example, and arrange for the collection and storage of a
sufficient volume of rainwater as it runs from the rocky surface of the
plateau. We may do this by restoring to use the old reservoirs; or we
might possibly achieve our end by damming some of the rocky gullies
which bring down the run-off from the plateau to the plain. Now that the
artesian idea is shown to be out of the question, there is justification
for a thorough investigation as to the best method of collecting and
conserving the local rainfall.
7. _Are the Artesian Water Supplies of the Oases diminishing?_
The native cultivators in certain parts of Kharga and Dakhla have for
some years past found that their wells no longer discharge at so high a
level as formerly, and in consequence some of their land has gone out of
cultivation. From this fact, and from the evidence of the former greater
prosperity of the oases which is afforded by the various ruins of
temples, forts, and villages, by the large areas of formerly cultivated
lands, and by numerous sanded-up wells, it has sometimes been inferred
that the total yield of the oasis-wells is now but a fraction of what it
formerly was. But, as Mr. Beadnell has pointed out,[24] the remains of
the past which exist in the oases belong to successive generations, so
that we cannot fairly draw such a conclusion from them; and the
reduction or cessation of the discharge of certain wells does not
necessarily imply any falling-off in the total water-output of the
oases. Mr. Beadnell’s experiments on flowing wells in Kharga have
clearly shown how the opening of a new well at a slightly lower level
will affect the discharge of an old well, even one at a distance of a
kilometre or more, by lowering the static head in its vicinity.[25] And
since a large number of wells have been bored in recent years both in
Kharga and Dakhla, it is most likely that the discharge from these wells
has caused a falling-off in the yield of older ones situated at slightly
higher levels. In this connection it will be well to note that it is not
the mere _existence_ of a new well that affects the static head, but the
_discharge_ from it. If the new well is securely closed so as to
discharge nothing, it has then no effect on the static head and
therefore none on the neighbouring wells. But for this to hold, it is
important that the well which is closed should be closed throughout its
entire depth; it is not sufficient merely to close its mouth so that it
does not discharge any water on to the ground, for there may still be
rapid leakage somewhere in the bore (unless effectively cased) into
porous unsaturated underground strata. Owing to the rapid rate at which
iron pipes are corroded in the wells of the oases, leakage of this kind
is more likely to happen with an abandoned modern well cased with iron
piping and plugged near the top, than with old wells which were filled
up with clay and sand. These factors, the mutual interference of wells
and the importance of preventing underground leakage, especially from
abandoned wells, being now thoroughly understood, steps are being taken
towards ensuring that future sites for new wells shall be judiciously
selected, and that leakage and waste from abandoned wells shall be as
far as possible arrested.
The method of carrying out measurements of well-discharges in the oases
is so inaccurate, and the records of the past output so defective, that
it is not possible to gather from them whether the total yield of
artesian water is at present diminishing or not. Mr. Beadnell considers
it likely, however, that the general average water-pressure in the oases
has been very much reduced within the historical period, owing to the
long-continued exploitation of the artesian supplies.[26] The general
study of the Libyan Desert which I have made in the last few years
suggests that a gradual reduction in the static water-pressure in the
oases may possibly have been brought about by other agencies than the
exploitation of the water in the oases themselves.
The first and most important of these other agencies is the withdrawal
of artesian water by the Nile in the neighbourhood of Dakka. As
mentioned on p. 113, it is practically certain that sandstone beds
carrying artesian water are cut through by the Nile along a distance of
several kilometres in that locality, and the influx of artesian water
into the Nile may far transcend in quantity that removed by the wells
and springs of the oases. The Nile has probably deepened its channel in
this region by a few metres within historical times, and thus cut
through a greater section of the water-bearing beds. An increase in the
sectional area of the beds cut through would naturally mean an increase
in the quantity of artesian water passing into the Nile, and hence a
lowering of the static water-surface extending perhaps to the oases.
The second possible other cause operating to diminish the static head of
the artesian water of the oases is the progressive desiccation of a lake
which may once have occupied a part of the Qattara depression. As
mentioned on p. 110, the floor of this great depression, large areas of
which are 80 metres and more below sea-level, is partly covered by a
salt-marsh, which is so soft and watery that it can only be crossed at a
few places. The hundreds of great water-cisterns cut in the limestones
of the plateau to the north of the depression—cisterns most of which are
now dry—as well as other ruins along the coast indicative of a
considerable former population, seem to show that the rainfall in the
littoral region has within the historical period been greater than it is
at the present day. When the rainfall in the coastal region was greater,
there must have been more drainage into the Qattara depression, and what
is now salt-marsh was thus possibly once a lake of some depth. Assuming,
as I think is likely, that an underground water-connection exists
between the marsh occupying the bottom of the depression and the
artesian water of the oases, it is obvious that any progressive lowering
of the lake-level consequent on the change of climate must have lowered
the static water-surface in the country extending southwards towards the
oases. In the oases themselves the lowering of the static surface would
of course be much less than at the lake; but it is quite conceivable
that even in the oases the lowering may have amounted to the few metres
which would cause some of the older and higher-lying wells to cease to
flow.
As both the deepening of the Nile channel in Lower Nubia and the
desiccation of the Qattara depression are probably still slowly
progressive, it is possible that these causes may to some extent account
for any slow lowering of the static water-surface in the oases which may
be still going on.
8. _“Lost” Oases—“Zerzura.”_
Of all the questions asked by intending travellers in the Libyan Desert,
none is more frequent than that as to the most likely whereabouts of
undiscovered oases, and especially as to the possibility of finding the
mysterious “Zerzura, or Oasis of the Blacks.” Hitherto the only aid
which I have been able to render to such inquirers has been to acquaint
them with the various statements which have been made by Arabs at
different times as to the situation of Zerzura, with the routes which
have been followed by others (including myself) who have sought in vain
for it, and with the indications of old tracks which have been
encountered by these previous travellers. So contradictory have been the
various Arab statements, and so numerous the vain attempts to find the
place, that I have at times felt almost convinced that “Zerzura” is a
myth. But Owenat and Merga were little more than traditions until a year
or two ago, and I think there is a sufficient possibility of the
existence of undiscovered springs or oases to encourage a further look-
out being kept for them, more especially as a consideration of the
general surface-contours and static water-contours which are now
available may furnish a new aid in the matter by narrowing down the
field of search.
As regards Arab traditions concerning Zerzura, the earliest account of
them which I have been able to trace is that of Sir Gardner Wilkinson in
his ‘Topography of Thebes and General View of Egypt,’ published in 1835,
p. 359. Wilkinson’s book is now rather scarce, and his statement
concerning Zerzura is so short that I quote it in full:
“About five or six days west of the road from el Hez to Farafra is
another Oasis, called Wadee Zerzoora, about the size of the Oasis Parva,
abounding in palms, with springs, and some ruins of uncertain date. It
was discovered about nine years ago (_i.e._ about 1826) by an Arab in
search of a stray camel, and from the footsteps of men and sheep he
there met with, they consider it inhabited. Gerbabo, another _Wah_, lies
six days beyond this to the west, and twelve days from Augela; and
Tazerbo, which is still farther to the west, forms part of the same
Oasis; and they suppose that Wadee Zerzoora also communicates with it.
The inhabitants are blacks, and many of them have been carried off at
different times by the Moghrebins for slaves; though the “Vallies of the
Blacks,” a series of similar Oases, lie still farther to the west.”
In footnotes he adds:
“It is supposed that the blacks, who invaded Farafreh some years ago,
and kidnapped a great number of the inhabitants, were from this Oasis.
“By another account Zerzoora is only two or three days due west from
Dakhleh, beyond which is another _wadee_; then a second abounding in
cattle; then Gebabo and Tazerbo; and beyond these is Wadee Rebeeana;
Gebabo is inhabited by two tribes of blacks, the Simertayn and
Ergezayn.”
Particular interest attaches to Wilkinson’s account of the tradition,
not only because of its being the earliest, and therefore less likely to
be coloured by imagination than later versions, but also because of the
remarkable fact that although some of the other places named in the
above extract were unknown to European geographers at the time, they
have all since been discovered; “Gebabo” and “Tazerbo” by Rohlfs, and
“Rebeeana” by Mrs. Forbes and Hassanein Bey.
The weak point in the account is the loose Arab way of stating
directions. Gebabo (Kufra) is not _west_ of the Baharia-Farafra road,
but south-west. If we amend the bearing in the first account
accordingly, and take Zerzura at about midway between the Baharia-
Farafra road and Kufra, it must lie near the intersection of the
parallel of 26° with the meridian of 26°, or some 200 kilometres east of
the Mehemsa Hattia, and some 150 kilometres north-west of Rohlfs’
“Regenfeld” camp. In the second account given by Wilkinson, the words
“due west” suggest that the bearing is more certain. Two or three days
(say 120 kilometres) due west of Dakhla would put Zerzura in about
latitude 25½°, longitude 27½°, or about 30 kilometres north of Rohlfs’
“Regenfeld.” There is thus a difference of more than 100 kilometres
between the two positions indicated by the accounts given to Wilkinson.
Rohlfs evidently knew of the traditions regarding Zerzura before making
his attempt to reach Kufra from Dakhla in 1874; and since such an
intermediate oasis, if it really existed, would be an immense aid to him
in attaining his objective, he made careful inquiries concerning it
before leaving Dakhla. But he found that though every one in Dakhla knew
the names of Zerzura and Kufra, no one could tell him where Zerzura was,
nor how far it lay from Dakhla.[27] Evidently Rohlfs placed little faith
in its existence, for during his journey he gave the name “Zerzura” to a
locality, about 120 kilometres west-south-west of Dakhla, where he found
nothing but very sparsely scattered vegetation.[28] Ascherson, who was
with Rohlfs in Dakhla, was however informed by Hassan Effendi, one of
the principal inhabitants of Mut, that about a hundred years before
(_i.e._ about 1770) there had been frequent raids on Dakhla by Arabs
from the south-west; and that in order to stop these raids the Mameluke
Government of the time installed a military colony, called the Surbaghi,
in the village of Qalamun. These Surbaghi went out and destroyed all the
wells for seven or eight days’ distance along the road by which the
raiders had come. This road, which up to that time had served as a trade
route from Darfur, before the road through Kharga was opened, was in
consequence forsaken. The road was said still to exist, and to lead into
the desert from Mut. At a day-and-a-half’s journey from Mut there were
said to be two “pillars,” half an hour’s distance apart, which Hassan
Effendi’s guard likened to minarets. In the neighbourhood there were
said to be large stone-quarries, and about thirty years previously an
iron instrument had been found; the instrument was still in Hassan
Effendi’s possession, and some wonderful stories had grown up as to the
manner of its use.[29]
Ascherson himself believed this information of Hassan Effendi’s to be
substantially correct, but he thought the “pillars” might be merely
pinnacle-shaped natural rocks. I am likewise inclined to believe it
trustworthy, but I think the “wells” may have been merely water-
dumps—that is, collections of jars of water. My reason for this view is
twofold. In the first place, the ground-levels in that part of the
desert are so far above the static water-level, that if wells were bored
they would have to be very deep, and the water would not rise anywhere
near the surface (there can, of course, be here no question of local
rainfall as a source for the water in the “wells”); and in the second
place, it seems very probable that the accumulation of large broken
earthenware jars which I discovered in 1916 at the place I named
“Pottery Hill” (latitude 24° 26′ 27″, longitude 27° 38′ 54″) is one of
the dumps in question.[30]
The importance of the above interpretation of Hassan Effendi’s story, if
accepted, as I think it must be, lies in the proof which it furnishes
that there cannot possibly be an undiscovered oasis anywhere near a
point seven or eight days from Dakhla in the direction of Kufra. A
consideration of the static water-levels and the contours of the ground
would lead us to this conclusion if the views I have advanced on the
underground-water question are correct; but the fact that the raiders
would never have made such a large water-dump if water could have been
readily got from a well in the vicinity makes the conclusion almost
certain independently of my hypothesis, and thus incidentally tends to
support the latter. Is it possible that “Zerzura,” which I am told
signifies in Arabic a starling (but is commonly applied to any small
bird), is here a corruption of some other name derived from “zeer,” a
water-jar, and that instead of “the _oasis_ of the blacks,” Zerzura was
really “the _water-depôt_ of the blacks”?
Schweinfurth has recorded[31] a story which was told him in Kharga
Oasis, that in 1872 some Arabs of a Darfur caravan, who had missed the
road, found a small oasis about 1½ days’ journey to the west of Beris.
The oasis was said to contain pools full of wild geese, date-palms, and
a temple. People who went out of Beris to find the place returned
unsuccessful. I am, however, rather inclined to think that the story has
some basis of fact, because in 1898 I myself found springs and traces of
ancient conduits in the dunes north-west of Beris, and there may be
other water-sources beyond the point which I reached.[32]
A writer in the ninth edition of Murray’s ‘Guide to Egypt,’ published in
1896, gives the following different statements of Arabs as to the
position of Zerzura:
(1) Some days south of the Dakhla Oasis.
(2) Five days west of Farafra Oasis.
(3) Three days west of Dakhla Oasis.
(4) Two or three days west of Selima Oasis.
The first of these statements would correspond sufficiently well with
Bir Terfawi, which is about 280 kilometres due south of Dakhla Oasis;
the second would place Zerzura about in latitude 27°, longitude 26°; the
third corresponds with the second account given to Wilkinson; while the
fourth would place it far to the south-west, in latitude 21½°, longitude
28°, or rather less than halfway between Bir Terfawi and Merga.
Mr. Harding King, who in his journeys in the Libyan Desert in 1909 and
1911 devoted much attention to the collecting of native information
regarding its geography, thinks that “Zerzura” may possibly be only a
generic name applied to any mythical or undiscovered oasis.[33] He heard
the name applied to the following localities:
(1) Rohlfs’ “Sersura.”
(2) The “Egyptian Oasis,” said to have been seen by an Arab from the top
of a high black hill lying in the dune-belt ten long days by ordinary
caravan from Kharga. Another Arab is said to have seen what is possibly
the same place eight days somewhere to the south of Dakhla. Both these
accounts agree that the place is a large oasis lying at the foot of a
scarp and containing olive trees.
(3) A stone temple eighteen hours’ journey west of Gedida in Dakhla
Oasis.
The information given to Mr. Harding King as to the position of the
“Egyptian Oasis” would place it about in latitude 23°, longitude 28½°,
or only some 50 kilometres north-west of Bir Terfawi; in fact,
considering the vagueness of the information, it might correspond fairly
well with Terfawi itself, except that there are no olive trees at
Terfawi, nor is that place overlooked by any escarpment or hill. The
“high black hill in the dune-belt” may just possibly be one of those I
mapped near the farthest point I reached with Moore in 1916, in about
latitude 24°, longitude 26°, though this would imply a direction south-
west of Dakhla, not south. It is not likely to be Gebel Kamil, as that
mountain lies much nearer to Merga than to Dakhla.
The “stone temple,” according to the information given to Mr. Harding
King, would be about 80 kilometres west of Dakhla Oasis, in about
latitude 25½°, longitude 28°, or not very far from the second of the
positions indicated by Wilkinson for Zerzura. But I think it is likely
that the statement really refers to the “Deir el Hagar,” a well-known
temple ruin much nearer to Dakhla.
So much for the various statements as to whereabouts Zerzura _may be_.
Let us now inquire where it is _not_. On the map I have indicated the
principal routes followed by explorers of the southern and western parts
of the Libyan Desert during the last fifty years. We may be tolerably
certain that Zerzura is not on, or very close to, any of these routes;
for although a depression within a kilometre or so of one’s track may
easily be passed without notice, the existence of a large oasis, such as
most of the traditions make Zerzura out to be, would almost certainly be
betrayed to an explorer by animal-tracks leading to it from considerable
distances, except possibly in places where the ground was very sandy.
[Illustration: _Pottery Hill from the south: at foot, petrol and water
supplies of Prince Kemal el Din’s Expedition of 1923_]
[Illustration: _Jars at northern foot of Pottery Hill found by Prince
Kemal el Din in 1923_]
[Illustration: _Jars, worn away by sand-laden winds, found by Dr. Ball
in 1917 at southern foot of Pottery Hill_]
There is another method now available to us by which we may narrow down
the search for Zerzura or other “lost” oases; and that is, by a
consideration of the general contours of the country and those of the
static water-surface. Every oasis in the Libyan Desert must owe its
existence to one or other of two conditions: either it must depend on
springs fed by local rainfall, in which case, like the oases of Owenat
and Arkenu, it probably lies near to mountains of considerable height;
or else it must depend on underground supplies, and must therefore lie
in a depression wherein the ground-level and static water-level are
practically coincident, as in the cases of Kharga, Dakhla, Farafra,
Baharia, and Siwa. It is highly doubtful whether any mountain masses at
all approaching the altitude of Arkenu and Owenat can remain
undiscovered in any of the various areas in which Zerzura has been
traditionally placed. Zerzura is immensely more likely to be in a
depression, and indeed Wilkinson’s name _Wadi_ Zerzura almost
conclusively points to that view. The depression is more likely to be
shallow than very deep, for it presumably lies in the sandstone country
of the south-west, and all the known depressions of great depth are
confined to the limestone country of the north-east. It is quite easy to
trace out on our new map the areas wherein the static water-surface
would be reached by a depression of say 50 or 100 metres below the
general ground-level as indicated by the contours. We must, however,
remember that our ground-contours are liable to be considerably in error
in areas where observations have been few—that is, in precisely those
areas where the depression, if it exists, is most likely to be found. So
we must allow a good margin for our depth, and I have chosen 100 metres
on this account. On the map I have drawn the “locus” of points in the
southern part of the Libyan Desert where the static water-surface is 100
metres lower than the general ground-surface[34]; and I have edged with
red the only areas in the west and south in which the two surfaces are
within 100 metres of each other—that is to say, the areas within which
comparatively shallow depressions with underground springs _must_ lie,
if our contours of the two surfaces are drawn with even approximate
correctness. It will be seen from the map that this restricts the search
very considerably. In fact, if Zerzura is within the frontiers of Egypt,
it lies in all probability either to the west of longitude 26° 20′ and
north of latitude 26°, or to the east of longitude 27° and south of
latitude 23° 30′. We may dismiss from our field of search all the broad
tract of rising ground which extends from near the Dakhla escarpment
south-westwards towards Owenat; for unless Zerzura is in a depression of
great depth, it cannot possibly lie within this tract. This
consideration shows that most of the previous rough estimations of the
possible position of Zerzura must have been erroneous; and indeed it is
remarkable how few of the various explorers’ tracks traverse the only
two areas within which Zerzura, if it exists, almost certainly lies. The
northern area has only been crossed by the Rohlfs expedition of 1874,
and that near its eastern edge; while the western part of the southern
area has only been crossed by Prince Kemal el Din’s expedition of 1925.
Of all the Arab traditions, only those which would place Zerzura far to
the west of Farafra, or far to the south-south-west of Dakhla, can now
have any high degree of credibility. It can hardly, I think, be doubted
that the various traditions refer to more than one place; and in view of
the almost totally unexplored state of the only two areas in which our
new contour-maps indicate the possibility of unknown oases existing, it
is quite conceivable that at least one oasis may lie within each of
them. In the northern area, Siwa would probably make the best starting-
point from which to conduct a search, as it is easily reached by motor-
car from Alexandria or Cairo, and exploratory journeys southwards from
Siwa would mostly lie along the direction of the dune axes. In the
southern area, Terfawi or Sheb would form the best base for exploration;
at each of these places there is a good water-supply. I am inclined to
think that the southern area is more likely to yield results than the
northern one. Water was found to exist at a point some 15 kilometres
west of Terfawi, and though no other source was observed on the way to
Owenat, it was impossible to see very far on either side of the track,
so that such a source might have been passed within a few miles without
notice. Terfawi itself is very difficult to find, being inconspicuous
even when one is fairly close to it; and as it is in a sandy area,
tracks leading to it are soon obliterated. Another factor which favours
this southern area is that, the country being all Nubian sandstone, the
depth from the surface to the actual water-bearing beds (as
distinguished from the depth to the static water-surface) is certain to
be much less than in the northern area, where Cretaceous strata may
overlie the sandstone; hence there is more likelihood of the existence
of natural springs in the south than in the north.
9. _Can Travel in the Great Tracts of the Libyan Desert which are now
Waterless be facilitated by the Sinking of New Wells?_
It has sometimes been asked whether tracts which it is now difficult or
impossible to cross by camel, owing to the non-existence of wells or
natural springs within them, might possibly be opened up to transport by
the sinking of new wells along a proposed route, as has in fact been
done by the Senussi in the case of the well at Sarra, between Wanianga
and Kufra. Hitherto there have been no data from which to form an
opinion on this question; but a study of the new map will enable at
least a qualified answer to be given.
To be a practicable proposition, a new well must fulfil two conditions:
the first, that the boring must not be required to descend to a very
great depth; and the second, that the water when struck must rise in the
bore to within a reasonable distance, say 20 to 30 metres, of the
ground-level at the place. The first of these conditions will be
satisfied if the geological horizon of the water-bearing bed is
comparatively near the surface; the second, if the level of the static
water-surface in the locality is within 20 or 30 metres of the ground-
level.
As regards the first of these conditions, we know that the water-bearing
beds underlying the Libyan Desert are situated within the geological
formation called the Nubian sandstone. We may therefore eliminate from
our consideration all tracts in which this formation is overlain by any
great thickness of younger rocks, for all these rocks will have to be
passed through in order to tap the water-bearing strata.
Concerning the second condition, our map at once informs us that the
areas in which the static water-surface is within 20 or 30 metres’ depth
below the ground-level are decidedly limited. Even if we suppose that
there may be unknown depressions extending to a depth of 70 metres below
the general level of the country indicated by the contour-lines, we see
that the localities in which the second condition would be fulfilled are
confined to the tracts edged with red on the map. Outside these tracts,
not only have we no chance of discovering oasis-depressions, but we also
have no prospect of being able to bore wells in which water would rise
to within a reasonable distance of the ground-level. Thus the broad
tract of rising ground which extends south-westwards from Dakhla to
Owenat must always remain a waterless waste.
In the Egyptian portion of the Libyan Desert there are, as will be seen
from the map, only two areas in the west and south in which new wells
might successfully be bored: one extending for some 300 kilometres or so
to the south of Siwa Oasis in the neighbourhood of the western frontier,
the other extending for some 400 kilometres westward from the Nile in
the neighbourhood of the southern boundary. Fortunately, however, these
areas are so situated that wells sunk within them might be of
considerable use in opening up the outermost parts of Egypt to
exploration. A well near the western frontier about on the parallel of
26°, for instance, might make it just possible for cars or camels to
reach Owenat directly from Siwa, since it would divide the present
waterless stretch of 830 kilometres into two stretches of about half
that length; while one or two wells near the southern frontier between
the meridians of 27° and 28° would render Owenat fairly accessible to
camels coming from the Nile _viâ_ Terfawi or Sheb, and might be of great
use to explorers or geologists desirous of making a detailed
investigation of the Owenat and Arkenu region. From the little I saw of
Gebel Owenat during my visit to it in 1925, its geology must be of
extreme interest. The south-western part of the mountain appears to be
entirely composed of granite and other crystalline rocks, while the
eastern part presents huge cliffs of sandstone, with crystalline rocks
showing only at the foot; there is evidently a great fault traversing
the mountain mass, with a downthrow to the east; and the thickness of
Nubian sandstone exposed on the eastern precipices is greater than that
at any other place I have seen.[35] Gebel Kissu probably resembles the
south-western part of Owenat in being entirely composed of crystalline
rocks.[36] East of Gebel Owenat there are many igneous hills, some of
which exhibit bands of a dark brown colour. These bands, though probably
mostly igneous dykes, may possibly in some cases be the gozzany outcrops
of veins containing metallic minerals; I had no opportunity of examining
them at close quarters, but I saw enough to make me long for facilities
to undertake such an examination.
It may be remarked that a well in the northern area would probably have
to be very deep, owing to the comparatively high geological horizon of
the surface rocks there; but in the southern area, where the Nubian
sandstone forms the surface rock, no great depth of boring would be
likely to be required in order to tap the water-bearing beds.
Apart from the question of new wells in the more remote parts of the
Libyan Desert, a study of the map gives us some hints which may be of
value should it ever be desired to sink wells in places nearer to the
oases and to the well-known tracks. There appears, for instance, to be
no reason why wells should not be successfully bored at intervals along
the Darb el Arbain between the south end of Kharga and Lagia, or on the
south side of the Qattara and other depressions of the northern part of
the desert. In the southern area, the best sites to select for wells
will be depressions wherein the ground-level approximates most closely
to the static water-level; and readings of an aneroid barometer,
compared with corresponding readings at a place of known altitude, such
as Sheb or Terfawi, would enable the most favourable sites to be
determined. Observations of the geological structure will also be of
importance; for anticlinal folds, by bringing the water-bearing strata
nearer to the surface, would diminish the depth to which borings would
have to be carried to tap the beds; while faulting might likewise
introduce favourable conditions by producing cracks and fissures along
which the water could rise. It may be remarked that tamarisk-bushes are
generally a sign that water exists at no great depth. In regard to wells
in or near the northern depressions, it is obvious that sites on the
south side of the lakes and salt-marshes should be selected; for not
only is the artesian static level higher in the south, but the water is
less likely to be contaminated by salts derived from the lake and
marshes.
10. _The “Tortoise Marshes” of Ptolemy._
The passage in Ptolemy’s ‘Geographia’ (lib. IV. cap. 6, sect. 4) in
which the position of the “Tortoise Marshes” is given may be translated
as follows[37]:
There are two great rivers running into the Mediterranean; one of them
is the Gir, joining Mount Usargala with the Garamantic narrows, from
which, changing its course, the river is located in long. 42°, lat.
16°, and makes the Tortoise Marshes (Chelonitides Paludes), whose
position is long. 49°, lat. 20°.
The information on which Ptolemy based this statement, at least as
regards the river _Gir_, must have been very fragmentary, if not,
indeed, grossly erroneous; for there is no river flowing to the
Mediterranean anywhere near the positions he gives. But there can be
little doubt that his names _Gir_ and _Chelonitides Paludes_ refer to
real places, and there has been much speculation among geographers as to
their identification.
Thus, for instance, Knoetel[38] suggested that the _Gir_ may have been
the Wadi Djedi, to the south of Biskra, and the Tortoise Marshes the
modern Lake Melghir (lat. 34°, long. 6°)[39] Dr. William Smith[40]
thought that the _Gir_ was really a branch of the Niger, and the marshes
the modern Lake Fittri (lat. 13°, long. 18°). On the map of Africa on
the 2-million scale published by the Geographical Service of the French
Army in 1899 the Tortoise Marshes are depicted as lying in about
latitude 19° 20′, longitude 27°, with a note stating that this
delineation is taken from an earlier map of the Nile Basin by Miani, who
regarded the marshes as being connected with the “dry river” which was
then supposed to run northwards through Dakhla Oasis. Colonel Tilho has
lately suggested[41] the lowlands to the north-east of Lake Chad (lat.
18°, long. 17°) as a possible site for the marshes; while still more
recently Mr. Harding King[42] has thought that they might perhaps be
identified with the salt lake of Merga (lat. 19° 3′, long. 26° 19′).
Of the various localities which have been suggested as possible sites
for the Tortoise Marshes, the only one which I have visited is Merga. I
found the salt lake at that place to be very small, covering only some
10 acres; it lies at an altitude of 509 metres above sea-level, and
though it is situated in a rather wide shallow depression, I saw no
traces of any extensive salt-marshes around the lake, while the
configuration of the surrounding country appeared to me to be such that
the depression cannot possibly have formed part of a continuous
drainage-channel.
I do not know whether it has hitherto been suggested that Kufra Oasis
may be the site of the Tortoise Marshes; but on correcting Ptolemy’s
figures for the errors in his adopted position for Alexandria and in the
length which he assumed for a degree of latitude, I find Kufra is in
very much closer agreement with them than any of the places named above.
In Ptolemy’s day, even the latitudes of but few places had been
astronomically observed (Alexandria, where he himself resided, was
supposed by Ptolemy to be in latitude 31°, instead of its true 32° 12′),
and as the only method at that time known for astronomically determining
differences of longitude was by the observation of eclipses, the number
of observed longitudes was smaller still. The process by which Ptolemy
deduced his position for the Tortoise Marshes and other places in the
interior of Libya was most probably that of first estimating their
distances south and west of Alexandria from travellers’ itineraries,
then converting these distances into degrees of latitude and longitude,
and finally subtracting the differences thus found from the latitude and
longitude of Alexandria. But Ptolemy made the great mistake of assuming
the length of a degree of latitude (or of equatorial longitude) to be
500 stades, instead of the 700 stades which it really is. Thus all his
dead-reckonings resulted in differences of latitude and longitude which
were too great in the proportion of seven to five. If we correct
Ptolemy’s position for Alexandria, and his dead-reckoning for the
erroneous assumption which he made regarding the size of the Earth, as
follows:
_Latitude._ _Longitude._
Ptolemy’s position for Alexandria 31° 60° 30′
Ptolemy’s position for the Tortoise
Marshes 20° 49°
------- -------
Ptolemy’s difference of lat. and long. 11° 11° 30′
These differences reduced in the
proportion of 5 to 7 become, in true
degrees and minutes 7° 52′ 8° 12′
The true position of Alexandria is 32° 12′ 29° 53′
------- -------
Whence the corrected position for the
marshes in our modern system becomes 24° 20′ 21° 41′
we get lat. 24° 20′ and long. 21° 41′ for the Tortoise Marshes.
Comparing this position with that of Kufra, we find that the latitude
agrees very closely with the 24° 14′ which Hassanein observed at Taj,
the principal village of that oasis; while the longitude, though more
than a degree and a half west of that of the principal village, is
almost exactly correct for Taiserbo, the north-western oasis of the
Kufra group.
Besides this remarkably close agreement as regards position, Kufra
presents several natural features which would tend to support the view
that it may be the locality which Ptolemy meant by the “Tortoise
Marshes.” Not only is Kufra an extensive tract of relatively low-lying
ground with numerous lakes and salt-marshes of very considerable size,
but it has distinctly the form of a valley. Hassanein Bey repeatedly
refers to it as a valley in his description[43]; Mrs. Forbes also speaks
of the “Wadi of Kufra,” and mentions that as a result of a ride westward
from Taj the “wadi” was found to have no definite ending to the
west.[44] What more natural, therefore, than that some ancient traveller
should have imagined that Kufra was a series of marshes formed by a
river coming from the south-west? And is it not possible that Ptolemy,
in endeavouring to piece together the scraps of information he could
get, may have mistakenly inferred that this river ultimately reached the
Mediterranean, and also have confused the account of it with those of
other streams further south, which may have been branches of the Niger?
11. _The Sand-Dunes._
The sand-dunes of the Libyan Desert have been the subject of careful
studies by Mr. Beadnell and Mr. Harding King, and practically all that
was known concerning them up to the outbreak of the Great War is
contained in the excellent papers by them which were read and discussed
at meetings of the Royal Geographical Society in 1910, 1916, and
1918.[45]
With the commencement of the Senussi campaign in Egypt in 1915, the
sand-dunes sprang into new and unexpected importance, from the fact that
they formed one of the principal hindrances to the free movement of
troops, and more especially of military motor transport, across the
desert. Every line of dunes of any considerable extent had now to be
carefully mapped and examined for possible car-passages through it; the
result was to add greatly to our knowledge of the distribution and
extent of these remarkable features of the Libyan Desert, especially in
the northern parts, where many long lines of dunes were found of which
the existence was previously unsuspected by geographers, though they are
familiar landmarks to the Bedouin of the region, and all except the
smallest bear Arabic names. After the cessation of hostilities, interest
in the distribution of dunes was maintained, because of the increasing
use of motor-cars in place of camels for transport across the desert;
and this circumstance has recently led to light being thrown on the
distribution of the dunes in those southern regions which lay outside
the field of operations during the war.
It is rather curious to note that although the sand-dunes form the
greatest obstacle to motor-cars in the desert, yet it is chiefly by
means of motor-cars that the true extent and distribution of the sand-
dunes has been ascertained. Dunes are the most difficult of all desert
features to map properly by ordinary reconnaissance methods with camel
transport. Their smooth outlines provide no points on which
intersections can be made, and no survey marks put on them will remain
in place for more than a few hours, or at most a few days; they occur
mostly in nearly level country, where it is impossible to find a station
whence they can be overlooked; the absence of shadows on them renders it
impossible to say whether one is looking at a single line of dunes, or
at several lines, miles apart, one behind the other in echelon. The only
sure way of mapping dunes is to traverse their sides along their entire
length, and this is impracticable with camels owing to the enormous
distances which would have to be covered without water. But with motor-
cars one can run alongside them at 40 kilometres an hour instead of the
camel’s 4, and their distribution can thus be rapidly and easily
ascertained.
[Illustration: _Prince Kemal el Din’s expedition of 1925 leaving Bir
Terfawi for the waterless journey of 250 miles to Owenat_]
[Illustration: _The well at Bir Terfawi in the palm clump_]
[Illustration: _Jebel Owenat from the south: the triple peak (5635 feet)
in centre_]
[Illustration: _The lake at Merga located by Prince Kemal el Din’s
expedition of 1925_]
On the map I have shown the distribution of sand-dunes as far as it is
known at the present day. A comparison of this latest map with that
given by Mr. Beadnell in his paper of 1910 will show how large is the
number of dunes discovered and mapped in more recent years; and it will
be noticed how pronounced is the constancy of the general direction of
all the lines in the Egyptian portion of the Libyan Desert. Though no
new lines of dunes comparable in extent with the great Abu Moharik belt
have been added to the map, some of the newly discovered lines are of
very considerable length, and they show an even more remarkable ratio of
length to breadth than the Abu Moharik belt; the Ramak dunes, for
instance, extend from near Moghara south-south-eastwards in a straight
line for more than 100 kilometres, with only a single small break, and
their width nowhere exceeds 1 kilometre. Moreover, the linear
arrangement in a direction from about 20° west of north to about 20°
east of south is even more pronounced in the newly discovered northern
dunes than it is in those previously known farther south. In the “great
sand sea” of Rohlfs, to the west of Dakhla and Farafra, the same
directional arrangement of the individual lines of dunes is very
noticeable, at least in the part near “Regenfeld” where I have
penetrated it; and I found the same general direction to hold for most
of the lines of dunes which I crossed on the way from Terfawi to Owenat.
During the war I received a number of reports of dunes extending nearly
east and west, often in curved lines; but investigations on the spot
showed practically all these reports to be erroneous, the commonest
mistake having been that of sighting different lines of dunes from
different places under the impression that they were a single one.
The general south-south-easterly direction which is so marked a feature
of the lines of dunes in the Egyptian portion of the Libyan Desert is
not, however, preserved in the extreme south-west of the country. The
line of dunes which crosses the Egypt-Sudan frontier in longitude 25°
40′ has a direction of about 30° _west_ of south, and the dunes which
extend from near Gebel Arkenu to the west of Gebel Owenat run about 38°
west of south. From a point about 80 kilometres to the south-west of
Gebel Owenat, the dune-lines make a still further distinct bend to the
west, changing their direction by some 20° to about 58° west of south,
and this latter direction is maintained by the dunes in the
neighbourhood of Sarra Well.[46] According to the maps of Rohlfs and
Mrs. Forbes, the dunes in the sandy tract to the north-west of Kufra
follow approximately the same bearing as those near Sarra. The
suggestion has been made that this swinging round of the direction of
the dunes in the south-western part of the Libyan Desert may be due to a
deflection of the prevailing wind by the mountain-masses. It is almost
certainly caused by differences in the prevailing winds—in fact, the
dune-lines probably afford a very exact index to the general wind-
direction in the areas where they occur—but I am inclined to think that
the regional distribution of atmospheric pressure, rather than local
deflection by mountain masses, is the cause of the differences of wind-
direction. The dunes, especially those near Sarra and Kufra, extend into
localities which seem to me to be too distant from the mountains for the
influence of the latter on the prevailing winds to afford a satisfactory
explanation.[47]
In certain areas, especially in the south, as for instance on the Arbain
road from Kharga to Sheb, around Pottery Hill, and in the region between
Owenat and Erdi, there are sand-covered tracts which can hardly be
mapped as dunes, because there are no very marked crests. These tracts
are easily crossed by camels, but give much trouble with cars, and would
probably be classed as dunes by car-drivers; but they are really only
vast undulating fields of drifted sand. On the old Darb el Arbain slave
road between Kharga and Sheb, one travels for more than 100 miles over
drifts of this kind. No footprint of camel marks the track, footprints
being obliterated in an hour or two. But one is never in doubt about the
way, for it is indicated by the skeletons of countless camels which have
perished on the weary march.
As regards the relation between dunes and the relief of the ground,
there is a gradual rise in the general level of the country from north
to south of about 1 in 1000, so that the southern ends of most of the
Egyptian lines of dunes lie at higher altitudes than the northern ends;
but I am fairly certain that this slight general inclination has nothing
to do with dune-formation. The straightest and cleanest-cut lines of
dunes are found in the flattest and most open parts of the desert, over
which the wind can sweep without obstruction. When a line of dunes
encounters in its southward progression a sudden fall in the ground-
level, as for instance at the north end of Kharga Oasis and to the east
of Pottery Hill, it usually breaks up; sometimes, as in Kharga, to
resume its course as dunes in more open formation, and at other times,
as near Pottery Hill, to spread out into a rolling plain of drifted
sand. It is not uncommon for the south end of a line of dunes to split
up, even on level ground, and the same can be seen, though less often,
at the northern ends; but this is probably merely because the dunes are
lower near the ends of the lines, and the width of the individual dunes
is consequently less.
The great lines of dunes are probably extending southwards at a very
slow rate, under the action of the prevailing wind. In this connection
it is interesting to note that Arabs always speak of the south end of a
line of dunes as its head, and of the north end as its tail; just the
opposite of what one would instinctively call them from merely looking
at the map, but correct if the dunes are known to have a southward
progression. Mr. Beadnell found the average rate of progression of
isolated small dunes in Kharga to be 15 metres per annum; but he remarks
that large dunes move more slowly than small ones, owing to the greater
mass of material to be transported. It is interesting to make a rough
guess at the antiquity of the dunes from the length of certain of the
longer lines and an estimated rate of progression. The Abu Moharik belt,
for instance, has a total length of about 350 kilometres; hence, if its
tail has remained stationary and its head has advanced southwards at an
average rate of 10 metres a year, some 35,000 years must have been
occupied in its formation.
As regards the height of the dunes, the only one I have carefully
measured is that close west of Rohlfs’ camp at Regenfeld, which I found
to be exactly 30 metres. Some of the dunes in the Abu Moharik and Ramak
belts are considerably higher; but I doubt if any of the Libyan dunes
rise much over 50 metres above ground-level, except in the “great sand
sea” which commences some 14 kilometres to the north-west of Regenfeld,
where Jordan estimated them to attain 100 metres or more.[48] The top of
a high dune to the south of Melfa Oasis was found by triangulation to be
172 metres above sea; the level of the ground at its foot is unknown,
but is probably not very much above the sea-level, so that Jordan’s
estimate of the height of some of the dunes in the “sand sea” is
probably by no means an exaggerated one.
Much has been written about eddies in connection with dune-formation.
The only observations I have made in this connection have been when
endeavouring to place temporary marks on dunes to form survey-points;
and the observations seem to show that if eddies are artificially
created at the top of a dune, a rapid lowering of the dune-crest
results. At first I used to thrust a walking-stick or ranging-rod some 2
or 3 feet into the dune-crest. The stick or rod, though quite firmly
fixed at first, was always found after a few hours lying halfway down
the dune-slope. The same thing happened with tripods firmly pressed down
into the crest. When I was carrying out a little triangulation to
determine the position of Rohlfs’ cairn at Regenfeld in 1924, I had
occasion to place a mark on a high dune, and I used an empty 4-gallon
petrol tin filled with sand, bedding it well into the dune; but the next
day it had disappeared from the crest, and was found, as the other
things had been, halfway down the slope. These observations remind me of
Mr. Barclay’s description of the method of dealing with sand-
accumulations in the Peruvian Desert,[49] where, as soon as dunes appear
and threaten to obstruct a railway, the local inhabitants turn out and
scatter pebbles and stones on the dunes, and very soon the dunes are
gone, having been carried away owing to eddies produced by the wind
around the stones. This device for removing incipient dunes might, I
think, be tried along the Kharga Railway, where the screens put up at
Mr. Harding King’s suggestion have not been entirely successful in
preventing the encroachment of sand on the line. I am the more disposed
to think an experiment of this kind might succeed, because I have
noticed that at the gap in the Ramak dunes (discovered by Col. Partridge
and known during the war as “Partridge Gap”) there are great lumps of
silicified wood, which may have led to the gap being formed.
There are two other physical characteristics of dunes which I do not
remember having seen mentioned. One of these is their low thermal
conductivity. On a hot day, one has only to thrust one’s hand a few
inches into a dune to encounter cool sand, while on a bitterly cold day
one can warm one’s hands by the same action; thus while the diurnal
variation of temperature of the surface of a dune is frequently very
great, it is almost insensible at quite a shallow depth in the interior.
It follows that diurnal expansion and contraction of the grains
composing the dune must be confined to the superficial layers, and the
movements thereby caused may materially assist in consolidating the
dune, by causing a “packing” of the grains. The other characteristic I
have noticed is that the surfaces of the dunes always appear to be
harder underfoot in the cool of the early morning than in the heat of
the day. This may, of course, be an illusion owing to one’s being less
sensitive to fatigue in the early morning, when one’s body has been
refreshed by sleep, and when the air-temperature is comfortable; but I
have often fancied there is more in it than that, and it may easily be
that the “packing” which has taken place owing to the considerable fall
of temperature in the night, to say nothing of the action of dew, may
have caused a real compacting of the surface layers.
In his paper of 1910, Mr. Beadnell considered that the sand composing
the dunes must be derived from arenaceous formations to the north, and
he made special mention of the need for a careful survey of the region
in which the dunes originate. A reference to the map will show how
correct was Mr. Beadnell’s surmise, at least in the case of the Egyptian
dunes which he had studied. The dunes originate in the great depressions
which stretch from Siwa to the Wadi Natrun, where, as Mr. Beadnell
inferred, the supply of arenaceous material from the loosely compacted
Miocene and Oligocene beds is almost inexhaustible. We have here one of
the clearest possible examples of the powerful action of the wind as an
excavating and transporting agent. The Qattara depression has been
excavated, largely by the wind, to a depth of over 130 metres below sea-
level, and the excavated material has been carried southwards, some of
it for nearly 1000 kilometres in distance and more than 500 metres in
height, to form the dunes. We see not only the excavated hollow, but
also the transported and piled-up arenaceous material removed from it;
and the total quantity of rock thus removed and re-deposited must amount
to hundreds of cubic kilometres.[50]
It is noticeable, however, that to the north of the “great sand sea” of
Rohlfs, which extends for an unknown distance westward, the known
depressions are much shallower; and yet the total quantity of sand in
this region is probably greater than that of all the more easterly dunes
put together. The country between Jarabub and Jalo is however almost
unexplored, though it is known to be very sandy and to be the theatre of
violent sandstorms. Whether similar deep depressions to that of Qattara
have been excavated along, or to the north of, the Jarabub-Jalo road, or
whether the area of loosely compacted arenaceous rocks exposed there is
so enormous as to have yielded such a quantity of sand without any very
deep depressions being formed, is a point that can only be cleared up by
further exploration in Cyrenaica.
We now come to the most puzzling of all the questions connected with the
sand-dunes, namely, why do they persist as sharply defined long narrow
lines in certain places, while the ground elsewhere is almost absolutely
free from sand? As Mr. Harding King puts it, the dunes appear to have a
curious power of collecting all the sand in the neighbourhood.[51] The
sheltering effect of ripples in the dunes, and a “shepherding” effect of
winds a point or two off the normal, have been suggested as possibly
affording some explanation of this apparent collecting-power,[52] but
neither of these suggestions appears to be adequate. I think a more
direct explanation is possible; namely, that the dunes really _have_ the
power of attracting sand from the air, and that this power is due to the
well-known law of attraction between a conducting surface at zero
potential and an electrically charged body. That air-borne sand-grains
in the neighbourhood of a dune may carry electric charges was shown by
Mr. Harding King’s experiments, recorded in his paper of 1916, and I
have since found sand-grains to carry electric charges of high voltage
during sandstorms in Cairo. As to how the grains become charged with
electricity, it can hardly be due to friction against each other, for
the sand is remarkably uniform in composition, consisting generally of
rounded grains of almost pure silica coated with a thin translucent film
of ferric oxide,[53] and for the production of frictional electricity it
is generally regarded as necessary that the two bodies which are rubbed
together should be of dissimilar materials;[54] nor is it likely that
they are electrified by friction with the air, for they travel with it.
But if the particles are carried into an upper air-stratum, they will
gradually acquire its potential by conduction; and the ordinary
potential gradient in the air in Egypt is over 100 volts per metre of
height.[55] Hence a sand-particle carried along for some time in the air
at a height of only 20 metres above the ground may become charged to 7
electro-static units of potential, or 2100 volts; and on approaching a
dune rising to about that height it will be attracted to the dune, which
being in connection with the earth is at zero potential. The attraction
will be very small unless the sand-grain approaches very closely to the
surface of the dune; from some calculations I have made, a spherical
grain of silica half a millimetre in diameter charged to a potential of
2100 volts will not be attracted with a force equal to half its own
weight until it approaches within a centimetre of the conducting
surface, and the grain will have to approach within 7 millimetres of the
surface before the attraction is equal to the weight of the grain. But
the important thing to note is that the attraction is independent both
of the sign of the charge and of the direction of the wind which carries
the particle. Another way of looking at the matter is to consider the
probable distribution of the equipotential surfaces in the air about a
dune, as in the diagram below, which represents a section taken
perpendicular to the direction of the wind and to the axis of the line
of dunes:
[Illustration: _Diagram showing suggested distribution of equipotential
surfaces and directions of lines of force in the neighbourhood of a
sand-dune_]
The equipotential surfaces, shown by dotted lines, will be squeezed
together over the dune, and the electric forces, shown by full lines and
arrows, being everywhere normal to those surfaces, will converge to the
dune. Thus the dune will attract any particles carried by the wind and
charged by conduction to the potential of the air conveying them. If the
wind changes its direction, the attraction will still persist unaltered.
Another point worth mention is that a sand-dune may be far more
effective in its attraction of sand than a rocky ridge of the same size
and shape would be; for when the electrified grains approach the surface
of the dune, uncharged grains can rise from the dune to meet them;
possibly some of the dancing of sand-grains which is frequently
observable along the crest of a dune during a wind may be due to this
cause.[56] During violent sandstorms, particles of sand may be whirled
aloft and kept suspended at high altitudes for a period long enough for
them to acquire very high potentials, and a sudden cessation of the
storm may cause them to fall so rapidly that much of the charge may be
retained; in which case, of course, electrical attraction may cause a
very considerable deviation from the purely gravitational paths of the
falling particles, the deviation being always towards the dune. Since
the closeness of the equipotential surfaces and the mobility of the
sand-grains composing the dune are greatest along the dune-crest, the
maximum rate of deposition will be at the top of the dune, which is also
the place where the wind exerts its greatest action in conveying the
sand of the dunes southward. That the long axes of the lines of dunes do
really correspond with the direction of the prevailing wind is certain,
both from Mr. Beadnell’s observations of wind-direction[57] and from the
fact that the scoring of the limestones in the Libyan Desert by wind-
borne sand coincides in direction with the lines of dunes.[58] Thus I
suggest that while the extension of the lines of dunes southwards is
purely the result of the prevailing north-north-west wind, their clean-
cut character and narrow width is the consequence of lateral attraction
by the dunes themselves on the flying electrified particles of sand.
If the hypothesis I have ventured to put forward is regarded as likely
to furnish the true explanation for the formation of the lines of dunes,
I hope that someone possessing the necessary skill in electrostatic
measurement will put it to a crucial test, by carrying out observations
on and near the dunes themselves. If a number of flying sand-grains
could be caught from different levels in an insulated metallic receiver
connected to an electrometer, the readings of the electrometer would
enable the potentials to which the grains were charged at different
levels to be calculated, provided the electrostatic capacity of the
receiving system and the number and diameter of the grains giving up
their charge were measured. Allowance would, of course, have to be made
for any electrification of the receiver by the air, or by uncaught sand-
grains, as well as for losses by imperfect insulation of the collecting
system. In a preliminary experiment of this kind which I made in Cairo
during a sandstorm, leakage from the electrometer was found to be so
great as to vitiate any attempts at accurate measurement; but I think
this particular difficulty could be overcome with a specially designed
apparatus, or that at least the leakage could be measured and allowed
for. The taking of observations of any kind in dune areas during windy
weather is a most difficult and trying operation, and commonly means, in
addition, a sojourn of some weeks in isolated and otherwise
uninteresting areas. But the advent of the motor-car has so greatly
facilitated communication in the deserts that difficulties of access are
now not so great as they used to be; the south end of the 35-kilometre
line of the Kattania Dunes, for instance, about 90 kilometres west-
south-west of Cairo, which would form an excellent site for detailed
observations, can now be reached by a four or five hours’ car journey
from Cairo instead of the several days’ journey by camel which was
formerly necessary.
12. _The Distribution of Stone Implements._
I have not made any special study of stone implements, nor have I been
able to pay any considerable attention to their collection during my
journeys, having usually found my time very fully taken up with other
matters. I have, however, been much struck by the wide distribution of
stone implements in the desert. I have found them, for instance, not
only in Siwa Oasis and near the wells of Abu Mungar and Sheb, but also
on the plateau between Baharia and Farafra, on the open desert between
Terfawi and Owenat, and to the south-west of Owenat near the Anglo-
French boundary. This wide range of occurrence, coupled with the finds
of Schweinfurth and others on the plateaux nearer the Nile, inclines me
to think that there is scarcely any part of the Libyan Desert in which
stone implements might not be found by an expedition which would make
the collection and study of them one of its main objects. The likeliest
places in which to search (besides the neighbourhood of old wells and
springs) would appear to be the shores of the various lakes and around
the feet of hills affording shelter from wind and sun; for it is in such
localities that I have most usually come across implements and pottery.
Grinding-stones, often with a sort of stone rolling-pin, unpolished
celts, knives, and arrow-heads are the principal forms of implement I
have met with. Many of the grinding-stones must have been carried for a
considerable distance, for they are made out of rocks which do not
naturally occur in the localities in which they are found.
Whether this wide distribution of stone implements would justify the
view of Blanckhenhorn[59] that primitive man lived on the desert
plateaux rather than in the Nile Valley must, I think, be settled by the
further collection of specimens and by a careful comparison of the forms
to be found in the two situations. In this connection I may remark that
the grinding-stones I have seen in the desert seemed to me to be very
similar to those used by the Nubians of the Nile Valley to-day for
grinding corn.
APPENDIX
SOME RECENTLY DETERMINED POSITIONS IN THE LIBYAN DESERT
In view of their possible utility in connection with future exploratory
surveys, I give below a list of some of the more important of the
positions which I have recently determined by astronomical observation
in the southern part of the Libyan Desert, with brief descriptive notes
on the places to which they refer. The observations were made whilst
accompanying H.R.H. Prince Kemal el Din Hussein on his exploratory
motor-car expeditions of 1923, 1924, and 1925. The longitudes of Qasr
Farafra, Abu Mungar, Pottery Hill, and Regenfeld depend on the transport
of a box-chronometer; the others on wireless time-signals from Europe.
The positions of peaks were found by triangulation from the actual
observation-spots. The altitudes of camps are from careful barometric
determinations; those of the peaks depend on trigonometric levelling
from the camps.
_Altitude
_Place._ _Lat. N._ _Long. E._ above sea.
Metres._
° ′ ″ ° ′ ″
Merga, Camp 600 metres S.W. of west 19 2 29 26 18 32 526
corner of salt lake. (Lake surface
is 17 metres below level of camp.)
Bir Terfawi, Camp close to well and 22 55 12 28 52 51 244
palms
Gebel Kissu, Summit 21 34 59 25 8 26 1726
Gebel Owenat, highest point 21 54 34 25 0 47 1907
„ „ remarkable triple peak 21 53 51 25 1 39 1718
„ „ S.W. peak 21 49 35 24 53 52 1450
Chunk Hill, Summit 21 52 46 25 13 56 985
Owenat, Camp 600 metres S.E. of 21 48 35 24 51 45 568
western spring, on plain at foot of
mountain
Owenat, Camp at mouth of gully, 21 53 8 25 7 58 626
about 800 metres S.S.E. of eastern
springs
Gebel Kamil, Summit (160 metres 22 16 31 26 38 11 800
above plain)
Regenfeld, Rohlfs’ cairn of 1874, 25 10 49 27 24 22 470
rediscovered 1924
Pottery Hill, Summit (39 metres 24 26 27 27 38 54 506
above plain)
Sheb, Camp close to well, 240 22 19 48 29 45 46 228
metres N.N.E. of fort
Mut, Government Rest-house 25 28 37 28 58 24 119
Abu Mungar, Camp at small ruin, 400 26 30 22 27 35 29 117
metres E.S.E. of well
Qasr Farafra, Camp E. of village, 27 3 26 27 57 52 90
200 metres S.W. of Tomb of Sheikh
Dakhil
_Merga._—To geographers the most important of the new determinations
will undoubtedly be that of the uninhabited oasis of Merga, which had
not previously been visited by a European, and of which the situation
could hitherto only be guessed at from Arab statements. Merga lies in a
shallow depression about 50 metres deep, broken by sandstone hills and
sand-dunes, and extending for some 20 kilometres north-east and south-
west. The salt-lake, near the centre of the depression, measures about
300 metres by 150. It is surrounded by tall rushes and sandhills except
at its south-western end. There are numerous date-palms, both near the
lake and at considerable distances from it, as well as acacia trees and
tamarisk bushes. The neighbourhood of the lake swarms with mosquitoes.
Good and plentiful water was got by shallow digging about 1 kilometre
south-south-east of the observation-spot, and could probably be obtained
by digging almost anywhere in the depression. It is possible that the
names Bidi and Tura el Bedai, shown with a question mark on some Sudan
maps, may refer to different spots within the same depression. Owing to
the plentifully scattered vegetation, the place cannot easily be missed,
either by travellers passing within several miles of the lake, or by
aircraft; but the landing of aeroplanes in the neighbourhood of the lake
might be difficult owing to the extensive sand-drifts.
_Bir Terfawi._—Scarcely less important than the accurate fixation of
Merga is that of Bir Terfawi, the farthest south-west of all the
Egyptian artesian water-sources hitherto known. It will be remarked that
this latest determination places the well some 22′ south and 15′ east of
the position which I had provisionally assigned to it from the rough
data furnished by Lieut. Moore’s traverse of 1916.[60] A knowledge of
the true position of Terfawi is specially desirable for a traveller who
wishes to reach it, owing to the absence of any conspicuous landmark
near it and to the sandy nature of the surrounding country, which causes
tracks to be soon obliterated. There are numerous sandhills covered with
tamarisk bushes around Terfawi, and a little grazing for camels. Besides
the well at which observations were taken, water was found by digging in
the sand at the foot of some tamarisk mounds about 13 kilometres farther
west, and it is probable that good and plentiful supplies could be
obtained at shallow depths near any of the other mounds. The palmtrees
at Terfawi are few and small, and are less conspicuous than the
tamarisk-bushes. These latter should enable the place to be easily found
by aircraft; but landing would require some caution owing to the
prevalence of drifted sand.
_Gebels Kissu and Owenat._—The peaks of these mountains (especially that
of Kissu, because of its isolated character and sharply marked summit)
will form useful points for the connection of future surveys, being
visible from very long distances.
_Chunk Hill_ is a prominent isolated hill, nearly conical and of dark
colour, which forms a good landmark in the broken country to the east of
Gebel Owenat. It rises some 335 metres above the ground at its foot.
_The Springs of Owenat._—Of the two water-sources of Owenat whose
positions are given, the western one is the better, and is moreover very
easy of access, being practically on the level of the plain which
extends southwards from the mountain mass, and easily discoverable by
the numerous animal-tracks converging to it; it is a pool among great
boulders, obviously fed by percolation through cracks and fissures in
the granite mountain which towers above it. The eastern water-source is
less easy of access; it lies about 1 kilometre up a stony gully, and
consists of a series of pools in the rocky floor of the gully, fed by
trickling springs at the level where the granite is overlain by
sandstone. The plain to the south of the western spring would form an
excellent landing-ground for aircraft.
Whilst in the neighbourhood of Owenat, I had hoped to re-determine the
longitude of Hassanein Bey’s camp of 1923 with the aid of wireless time-
signals, or at least by triangulation-connection to one of my
observation-points, because a really accurate fixation of the longitude
of a point about midway along his route from Jarabub to Furawia would
possibly furnish the means of slightly correcting the longitudes
assigned to Hassanein’s camps at Kufra and Erdi. I was unfortunately
prevented from carrying out the desired connection; but from a hurried
car-traverse which I made, skirting the western side of the mountain
mass, in the course of which I must have passed pretty close to the site
of Hassanein’s Owenat camp, I think that camp really lies about in
longitude 24° 49′, or some 5 miles to the west of where I had previously
calculated it to be from Hassanein’s traverse data;[61] and that in
consequence the longitudes assigned to Kufra and Erdi from the same data
may be some 2 or 3 miles too great.
_Gebel Kamil_ is a sharply pointed hill of dark sandstone, forming a
useful landmark between Terfawi and Owenat. It was visible from the east
for more than 40, or from the west for about 20, kilometres. The name
was given to it by Prince Kemal el Din in honour of his father, Hussein
Kamil, the late Sultan of Egypt.
_Regenfeld_, it will be remarked, was found to be very nearly in the
position assigned to it by Jordan in 1876,[62] and my estimation of the
level of the place is only 20 metres higher than Jordan’s. The
neighbouring dune I found to be 30 metres high, agreeing exactly with
Jordan’s measurement of fifty years previously; and as far as I could
judge, the situation of the dune relative to the cairn seems to have
remained unchanged through this long interval, showing that there has
been at any rate no great lateral displacement of the dunes. The iron
tanks left by Rohlfs were quite intact, in spite of their half-century
of exposure; they had become covered with a hard dark brown film,
apparently of magnetic oxide of iron, not rusted in the ordinary way, a
fact which speaks strongly for the dryness of the region. Empty wine-
bottles left by the Rohlfs expedition were frosted by the sand-blast
wherever they were exposed; but surprisingly little of the glass had
been removed in this way.
_Pottery Hill_ is the northern one of a pair of nearly conical dark
sandstone hills about 40 metres high, which are conspicuous from some
distance owing to their situation on a nearly level sand-plain. I gave
it its name from the numerous jars which I discovered at its foot in
1917.
_Sheb Well_ is a pool of fairly good water in a shallow excavation at
the foot of a clump of dom palms, half surrounded by sand-dunes and
tamarisk mounds, 240 metres north-north-west of a fort; the latter
occupies a fairly conspicuous position on a sandstone hill about 25
metres high. Another well, situated at the foot of a clump of dom palms
about 11 kilometres farther north, is called Bir Terfau; this should
not, of course, be confused with the Bir Terfawi already mentioned.
To the north of Sheb, on the Arbain road, are three other water-sources
which I visited in 1925 and of which I determined the approximate
positions by a carefully controlled car-traverse: Bir Kassaba, in lat.
22° 41′, long. 29° 55′; Bir Abu Hussein, in lat. 22° 53′, long. 29° 55′;
and Bir Murr, in lat. 23° 22′, long. 30° 5′. Bir Kassaba is a pool of
good water at the foot of a clump of palms; Bir Abu Hussein, which
likewise yields good water, is a small excavation in sand near a hill of
pink coarse-grained granite, and usually requires to be dug out afresh
by each traveller; Bir Murr, on a little plain surrounded by hills,
consists of several holes, with fairly good water, excavated in the sand
at the south side of the outcrop of a steeply inclined bed of speckled
calcareous sandstone.
I have not myself traversed the desert to the east of Sheb, but
according to an intelligent Arab who has recently made the journey from
Sheb to Dungul, there are only two spots between these places at which
water is obtainable; they are Bir Abu Seifa, a small well of good water
about 50 kilometres from Sheb, and Bir Haleifa, about 15 kilometres
farther on, where there is a fort and numerous wells. This latter place
must, I think, be the same as the Bir Nakhlai, of which the position
found astronomically by Colonel Talbot in 1893 was lat. 22° 29′ 1″,
long. 30° 19′ 36″.
_The Rest-house at Mut_, in Dakhla Oasis, is a low whitewashed building
of three rooms, situated on open ground a little to the south of the
village. It is conspicuous by reason of its isolation from other
buildings, and forms a convenient starting-point for car journeys to the
south-west of Dakhla, as well as for journeys to Kharga by the southern
track which was mostly followed by cars during the war.
_Abu Mungar_ proved to be very close to the position given by Mr.
Harding King’s observations of 1912.[63] To the south-east of Abu
Mungar, about halfway to Dakhla, are two remarkable hills forming
outliers of the plateau. These hills, which are good landmarks, were
first discovered by Mr. Harding King, and as they appear to possess no
native name, I have called them “King’s Hills” on the map. The farther
one of the two from the plateau lies in lat. 25° 58′, long. 28° 11′,
from compass-bearings which I took to it on a careful car-traverse in
1924. The other is about 2 kilometres farther north.
_Qasr Farafra._—The position of the village of Qasr Farafra which I gave
in my paper of 1919[64] is proved to be very nearly correct by the
latest observations. The main advantage of the new determination is that
it is referred to an easily identifiable landmark, the tomb of Sheikh
Dakhil, instead of to the ill-defined centre of the village.
_Levels in or near Baharia Oasis._—I think it may be well to record here
that owing to an unfortunate mistake about the datum to which the level
of the old military railway-terminus at B6 was referred, all the
altitudes given on pp. 10 and 11 of my above-mentioned Survey Department
Paper of 1919 require a correction of 19 metres to be added. At the time
when I triangulated Baharia in 1917, I was informed that the level found
by the military authorities for the terminus was measured from the sea-
level datum, and I employed the value given for it as my fundamental
level. But I have since ascertained that an arbitrary datum was employed
by the military engineers; and trigonometric levelling from the Nile
Valley has shown that this arbitrary datum was about 19 metres above
sea-level.
FOOTNOTES:
[Footnote 1: The use of motor cars for reconnaissances in the Libyan
Desert, initiated by the British Army during the Great War, has been
considerably developed in the last few years by Prince Kemal el Din
Hussein (son of the late Sultan of Egypt), who has made several long
exploratory journeys with a fleet of cars of the caterpillar type,
specially constructed so as to be capable of carrying considerable loads
over sandy tracts which were impassable to the Army patrols. The
expeditions organized and led by His Royal Highness have resulted in
important additions to geographical knowledge concerning the Libyan
Desert, especially the southern and western regions, which have hitherto
been practically inaccessible. I had the pleasure of accompanying the
Prince on three of his journeys, and am much indebted to him for thus
affording me the opportunity of making many observations which would
otherwise have been impossible to me. I have further to thank him for
kind permission to illustrate this paper by reproductions from a
selection of the excellent photographs taken by him during our travels.]
[Footnote 2: Kindly communicated by Mr. A. R. Boyce, Director of Sudan
Surveys.]
[Footnote 3: Printed by the French Army Staff, 1926.]
[Footnote 4: By way of comparison, it may be remarked that the area (at
the sea-level contour) of the Qattara depression is about four times
that of the Dead Sea and Jordan Valley depression; but the latter
(reckoning to the lowest point of the bed of the Dead Sea) is about six
times the deeper of the two.]
[Footnote 5: A similar ambiguity of meaning very commonly attaches to
Arabic geographical terms. Thus _beled_ may mean either a village or an
entire country; _gebel_, either a mountain or a desert; _gezira_, either
an island or a part of the Nile bank which is alternately covered and
uncovered by the rise and fall of the river; _bir_, which properly
signifies a well, is often used to denote a spring or a water-cistern;
_wadi_, though its proper meaning is a valley, is also sometimes
employed for a closed-in depression, as in the case of the Wadi Natrun.]
[Footnote 6: ‘Nouveaux Mémoires des Missions de la Compagnie de Jesus
dans le Levant,’ Tome ii. (Paris, 1717), p. 74. This little book
contains a map of Egypt, prepared by Father Sicard at Cairo in 1715, on
which the “Mer sans eau” is shown with “bateaux petrifiez” depicted
alongside it.]
[Footnote 7: See “Particulars concerning the Valley of the Natron Lakes,
and that of the Old Bed of the River” (with map) by Andreossi, General
of Artillery, in ‘Memoirs Relative to Egypt, Written in That Country
during the Campaigns of General Bonaparte in the Years 1798 and 1799, by
the Learned and Scientific Men who Accompanied the French Expedition’
(London, 1800), p. 270.]
[Footnote 8: See Rohlfs’ ‘Drei Monate in der libyschen Wüste’ (Cassel,
1875), p. 337; and Jordan’s ‘Physische Geographie und Meteorologie der
libyschen Wüste’ (Cassel, 1876), p. 214.]
[Footnote 9: ‘Zeitschrift der Gesellschaft für Erdkunde zu Berlin,’
1902, Tafel III.]
[Footnote 10: ‘Topography and Geology of the Fayum Province of Egypt’
(Cairo, 1905), p. 67.]
[Footnote 11: Since the above was written, the Professor of Geography at
the new University of Cairo, M. Lorin, has suggested that at a former
geological epoch the Nile flowed through Siwa Oasis and thence westward
so as to make Cyrenaica an island. (See ‘L’Egypte,’ published by the
Institut Français d’Archéologie Orientale, Cairo, in December 1926, p.
153.) M. Lorin does not give any grounds for this suggestion, and I know
of no evidence that would justify its acceptance. As will be gathered
from my remarks elsewhere in this paper, it seems to me far more likely
that Siwa and the other northern depressions were in recent geological
times regions of central drainage, than that they were local deepenings
of a single channel; and in regard to earlier geological periods we
possess too little information to form any conclusions as to drainage-
lines.]
[Footnote 12: See Sir Hanbury Brown’s ‘The Fayum and Lake Moeris’
(London, 1892), pp. 40-48 and 106-110; Sir W. Willcocks’ ‘Perennial
Irrigation and Flood Protection for Egypt’ (Cairo, 1894), also the same
author’s ‘Egyptian Irrigation,’ third edition (London, 1913); Sir W. E.
Garstin’s ‘Report on the Basin of the Upper Nile’ (Cairo, 1904),
Appendix I. pp. 6-9; and Beadnell’s ‘Topography and Geology of the Fayum
Province of Egypt’ (Cairo, 1905), pp. 16-24.]
[Footnote 13: ‘Report on the Administration of the Irrigation Department
for 1895’ (Cairo, 1896), pp. 25-28.]
[Footnote 14: ‘Report on the Administration of the Irrigation Department
for 1896’ (Cairo, 1897), p. 129.]
[Footnote 15: It may be remarked that even if a channel were to be cut
from Lake Mariut to the Wadi Natrun it would still be necessary to
continue pumping on a large scale from Lake Mariut to maintain the
present-day drainage. According to returns communicated to me by the
Irrigation Service, the quantity of water removed from Lake Mariut by
pumping during the last nine years has averaged nearly 650 million cubic
metres a year; and even on the most optimistic estimate of evaporation
and seepage the Wadi Natrun could not be expected to dispose of half
this quantity after the lapse of the few years which would be occupied
in filling it to a permanent level.]
[Footnote 16: The average discharge of the Nile past Cairo in a year is
67 cubic kilometres. The capacity of the Qattara depression at the sea-
level contour is roughly about 900 cubic kilometres.]
[Footnote 17: _Geogr. Journ._, 56 (1920), pp. 97-99 and 161-163.]
[Footnote 18: Lucas (‘Natural Soda Deposits in Egypt’ (Cairo, 1912), p.
15) regarded the water of the Wadi Natrun as being due to infiltration
from the Nile, mainly on the ground that he supposed the water visibly
entering the wadi (as small streams and trickles) to do so on the north-
eastern side. But this, I think, is a mistake, as it neglects
consideration of the large springs of warm fresh water which bubble up
in and near the salt-lakes themselves, and also the large wells of the
monasteries which lie south-west of the lakes.]
[Footnote 19: A far better site for this well would have been in the
depression some 20 kilometres to the north-west, the ground-level at the
lowest point of which is only 61 metres above sea; but at the time when
the site of the well was chosen the existence of this depression was
unknown.]
[Footnote 20: See Appendix III. in the second volume of Willcocks’
‘Egyptian Irrigation,’ 1913, p. 853.]
[Footnote 21: Schweinfurth (_op. cit._, p. 855) even thought it might
have percolated to Siwa, in spite of the great distance (480 kilometres)
and the adverse direction of the difference of level between the two
places.]
[Footnote 22: Mr. Beadnell (‘An Egyptian Oasis’ (London, 1909), p. 139)
estimates the total discharge of the Kharga wells at 53,000 cubic metres
per diem. We should not, I think, err greatly in estimating the total
output of all the Egyptian oasis wells and springs at about five times
this, or 250,000 cubic metres per diem. As the average daily discharge
of the Nile in Egypt is about 180 million tons, a local influx into it
several times as great as the total discharge of the oasis wells would
represent but an insignificant addition to the discharge of the Nile. I
think, however, that the influx of the warm water might possibly be
detected by careful thermometric measurements made in the river near its
banks at the time of low Nile.]
[Footnote 23: This was the lowest level recorded at the time when I drew
the static contours on the map. The further exploration of the
depression by Mr. Walpole in the present year has shown that at one
place the ground-level descends to − 134 metres; the situation of this
local deepening is, however, such that the drawing of the static
contours would not have been affected by its inclusion in the list of
adopted data.]
[Footnote 24: ‘An Egyptian Oasis’ (London, 1909), p. 156.]
[Footnote 25: “The Mutual Interference of Artesian Wells,” _Geol. Mag._
(London, 1909), pp. 23-26; also ‘An Egyptian Oasis,’ chap. x.]
[Footnote 26: ‘An Egyptian Oasis,’ p. 140.]
[Footnote 27: ‘Drei Monate in der libyschen Wüste’ (Cassel, 1875), p.
115.]
[Footnote 28: See Jordan’s ‘Physische Geographie und Meteorologie der
libyschen Wüste’ (Cassel, 1875), p. 204. Jordan states that there were
on an average not more than two to four plants per acre in this
locality.]
[Footnote 29: See Rohlfs’ ‘Drei Monate in der libyschen Wüste’ (chapter
ix. of which was written by Ascherson), p. 250.]
[Footnote 30: In my paper on ‘Recent Determinations of Geographical
Positions in the Libyan Desert,’ published in Cairo in 1919, I gave (p.
14) the approximate position of the hill as lat. 24° 28′, long. 27° 45′.
The position given above is from a more accurate determination which I
made on revisiting the place with Prince Kemal el Din in 1923. On this
latter occasion the excavations made by the Prince in the sand around
the foot of the hill revealed the existence of hundreds of additional
jars, many of them intact, set in regular order in the sand and
obviously forming a water-dump. The majority of the jars bore clearly
incised markings, which Prince Kemal el Din was able later to identify
positively as being tribal marks of the Tebus, the marks being in fact
exactly the same as those used by the Tebu tribes of the present day.]
[Footnote 31: See Jordan’s ‘Physische Geographie und Meteorologie der
libyschen Wüste’ (Cassel, 1876), p. 215.]
[Footnote 32: ‘Kharga Oasis’ (Cairo, 1900), p. 57.]
[Footnote 33: _Geogr. Journ._, 42 (1913), p. 283; also ‘Mysteries of the
Libyan Desert’ (London, 1925), p. 304.]
[Footnote 34: This “locus” is of course given by joining up the points
of intersection of the 100-metre water-contour with the 200-metre
ground-contour, the 200-metre water-contour with the 300-metre ground-
contour, and so on.]
[Footnote 35: It is possible that the “Nubian Sandstone” in South-
Western Egypt may represent more than one geological system, as has been
found to be the case in Sinai. Col. Tilho (_Geogr. Journ._, 56 (1920),
p. 259) records that the sandstone at Ennedi proved to be of Upper
Silurian age by the fossils found in it.]
[Footnote 36: Though we passed within 10 kilometres to the east of Kissu
on the return journey from Merga to Owenat in 1925, I could not observe
the structure of the mountain very clearly, owing to haze. But it
appeared to me to be a huge granitic intrusion rising through the gneiss
which covers a large area hereabouts. The sandstone over which we had
been travelling since leaving Merga gave place to gneiss about 40
kilometres before we came abreast of Kissu, and gneiss continued to be
the country rock until we came within 30 kilometres of Gebel Owenat,
when sandstone reappeared, at first capping low hills and then forming
the plain.]
[Footnote 37: There exists no published English translation of Ptolemy’s
‘Geographia.’ Müller’s edition (Paris, 1883-1901) gives Greek and Latin
texts, with a Latin commentary.]
[Footnote 38: Quoted in Müller’s edition of Ptolemy’s Geography, p.
739.]
[Footnote 39: For the latest delineation of this region, see the
hypsometric map in the ‘Atlas de l’Algérie et de Tunisie,’ published by
the Cartographic Service of the Algerian Government, Paris, 1924,
Fascicule II.]
[Footnote 40: ‘Manual of Ancient Geography’ (London, 1891), p. 311.]
[Footnote 41: _Geogr. Journ._, 56 (1920), p. 94.]
[Footnote 42: ‘Mysteries of the Libyan Desert’ (London, 1925), p. 303.]
[Footnote 43: _Geogr. Journ._, 43 (1924), p. 288.]
[Footnote 44: ‘The Secret of the Sahara—Kufara’ (London, 1921), p. 235.]
[Footnote 45: Beadnell, “The Sand-dunes of the Libyan Desert,” _Geogr.
Journ._ 25 (1910), pp. 379-395; Harding King, “The Nature and Formation
of Sand Ripples and Dunes,” _Geogr. Journ._, 47 (1916), pp. 189-209;
Harding King, “Study of a Dune Belt,” _Geogr. Journ._, 51 (1918), pp.
16-33, and Discussion, pp. 250-258.]
[Footnote 46: For particulars of the distribution and directions of the
dune-lines near Gebel Owenat, and between that mountain and Sarra, I am
indebted to Prince Kemal el Din Hussein, who devoted special attention
to observing them on his expedition to Sarra in 1926.]
[Footnote 47: It may be remarked that in the desert of Northern Sinai
the dune-lines follow curved directions, instead of maintaining a single
direction as in the Egyptian Libyan Desert, and this curvature of the
Sinai dune-lines has generally been thought to be caused by the
deflecting influence of the mountain masses of Moghara, Yelleg, and
Hellal on the wind; but here also there may be regional influences at
work.]
[Footnote 48: See Jordan’s notes on pp. 204 and 208 of his “Physische
Geographie und Meteorologie der libyschen Wüste,” Cassel, 1876.]
[Footnote 49: _Geogr. Journ._, 49 (1917), p. 55.]
[Footnote 50: The steep Miocene escarpment, some 200 metres high, which
bounds the Qattara depression on the north, probably produces a great
eddy in the wind sweeping southwards over it, causing a whirl round a
horizontal axis south of the scarp; this may be very influential both in
excavation and in lifting the sand to great heights. As a large portion
of the floor of the depression at the foot of the escarpment is formed
of a salty sludge (sabakha), it is likely that a considerable quantity
of finely divided salt crystals are carried up with the sand in summer,
and this may conceivably act to some extent as a binding material in the
dunes.]
[Footnote 51: _Geogr. Journ._, 47 (1916), p. 209.]
[Footnote 52: _Geogr. Journ._, 51 (1918), p. 252.]
[Footnote 53: Many years ago I discovered that this film can be removed
by treating the sand with warm hydrochloric acid, leaving the grains of
snowy whiteness (see my ‘Description of the First or Aswan Cataract’
(Cairo, 1907), p. 57). But I think traces of iron in the ferrous state
must exist even within the white siliceous body of the grains, and be
continually passing into the ferric state at the exposed surface; for
otherwise it is difficult to account for the persistence of the yellow
colour in spite of the mutual rubbing of the grains during their
transport by wind. The film may possibly have an influence on the
electrical behaviour of the grains, by reason of its possessing a higher
conductivity than the silica of the interior.]
[Footnote 54: Professor P. E. Shaw has quite recently (_Nature_, vol.
118 (1926), p. 659) suggested from laboratory experiments that the
mutual _impact_ of particles of _identical_ nature in sandstorms may
result in electrification of the particles and of the air with charges
of opposite signs. I think most travellers in the desert have, like
myself, formed the impression that electrification of the air is more
pronounced during sandstorms (khamsins) than at other times. But it is
curious that observations made with the electrograph at Helwan
Observatory do not seem altogether to bear this out. Dr. Hurst finds
that khamsins are usually accompanied by low values of potential
gradient in their early stages, with great disturbances both positive
and negative for some hours before and after the break of the khamsin
(‘Observations on Atmospheric Electricity at Helwan Observatory,’ Survey
Dept. Paper No. 10 (Cairo, 1909), pp. 52-62).]
[Footnote 55: Hurst, _op. cit._, p. 28.]
[Footnote 56: Mr. Harding King records in his paper of 1915 that he
found sand-grains blown off the dunes to be electrified; can these have
been grains which became charged by jumping up from the dune to meet
others and thus sharing their charge?]
[Footnote 57: _Geogr. Journ._, 35 (1910), p. 385.]
[Footnote 58: The hot and disagreeable south and south-west winds which
blow during the “khamsins” of the spring are much less constant in their
direction; and though they produce thick sandstorms, they have always
seemed to me to occasion rather a general scattering of the sand over
the surface of the desert than to form deposits having very definite
arrangement; moreover, they blow on relatively few days of the year. The
far more prevalent cool strong north-north-west winds, on the other
hand, often scarcely vary a point in direction for days together, and
their coincidence with the direction of the dune-lines is very
pronounced.]
[Footnote 59: _Zeitschrift der Gesellschaft für Erdkunde zu Berlin_,
1902, p. 760.]
[Footnote 60: ‘Recent Determinations of Geographical Positions in the
Libyan Desert,’ Survey Department Paper No. 34, 1919. p. 14.]
[Footnote 61: _Geogr. Journ._, 44 (1924), p. 377.]
[Footnote 62: Jordan’s final position was lat. 25° 11′ 7″, long. 27° 25′
0″. The longitude given on the document found in the bottle at the cairn
(14° 42′ E. of Berlin, or 28° 3′ 42″ E. of Greenwich) was a preliminary
value resulting from Jordan’s observation of a lunar distance, and was
wrong by some 40 miles.]
[Footnote 63: Mr. Harding King’s observations gave the latitude as 26°
30′ 46″, and the longitude as 27° 36′ (see his map in _Geogr. Journ._,
42 (1913), p. 516).]
[Footnote 64: ‘Recent Determinations of Positions in the Libyan Desert,’
Survey Department Paper No. 34 (Cairo, 1919), p. 12.]
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