TOPOGRAPHY AND GEOLOGY
Dimensions of the Oasis-Depression — Jebel Têr, Jebel Tarif,
and other Hills within the Depression — Aspect of the Oasis from
the surrounding Escarpments — Geological Sequence — Nature and
Thickness of the Strata — Geological History of the Oasis —
Formation of the Depression — Difference of Level of Strata on
either side of the Depression — The Great Longitudinal Flexure
— Height of the Floor compared with Sea-Level — Altitudes.
Kharga, the eastern of the two southern oases, is a depression lying
with its longer axis north and south, mostly bounded by steep and
lofty escarpments, but open to the south and south-west, on which
sides the country rises gradually from the floor of the oasis. The
extreme length of the depression, from the northern wall to Jebel
Abu Bayan, which for convenience may be regarded as the southern
limit of the oasis proper, is 185 kilometres, or 115 miles. The
general trend of the eastern escarpment is nearly due north and
south, but that on the west is very irregular, while to the south
and south-west there is no definite boundary. The breadth of the
depression may therefore be said to vary from 20 to 80 kilometres.
The ranges of Jebel Têr and Jebel Tarif form isolated hill-massifs
in the centre of the northern part of the depression, while Jebel
Ghennîma and Jebel Um el Ghennaim are conspicuous outliers of the
plateau on the east side. With the exception of these, the floor is
destitute of anything beyond comparatively insignificant eminences,
unless we include the small range of hills known as the Gorn el
Gennâh, to the south-east of the village of Gennâh, which is
noticeable more on account of its sharply-defined peaks than of its
general elevation above the surrounding country. Referring to the two
conspicuous peaks, Ghennîma and Um el Ghennaim—Jimmy and Jemima,
as I have heard them dubbed—reminds me that on the Survey and on
some of the older maps the names are reversed. I have questioned
a number of natives regarding the names of these hills, and have
invariably been informed that Ghennîma is the more northerly of
the two.
The villages, wells, and cultivated lands lie within a north and
south band, occupying the lowest portion of the floor, and following
the general trend of the depression. They are, however, broken up
by a broad area of barren desert into two distinct north and south
groups, of which Kharga and Beris villages are the chief centres
respectively. A description of these is reserved for a later chapter.
When a traveller, after crossing the broad monotonous plateau,
at length reaches the scarp or wall of the oasis, and sees spread
out before him a vast depression, stretching in some directions
as far as the eye can reach, in others to the opposite bounding
walls dimly discernible on the far horizon, he can hardly refrain
from speculating as to the causes which have given rise to such
huge hollows in the plateau. When he descends to the cultivated
portions of the floor of the depression, and sees those numerous
bubbling springs, which alone make life possible in the midst
of this otherwise deadly wilderness, his second inquiry is as
to whence comes such abundance of water in one of the most arid
regions in the world. These questions are worth asking, and, so
far as the present state of our knowledge permits, it will be my
endeavour to answer them. I propose, therefore, to briefly place
on record such information and data as I have been able to gain,
but as both topography and water-supply are intimately connected
with the geology of the district, it will be necessary at the outset
to devote a few pages to a consideration of the latter.
[Illustration: THE CHRISTIAN NECROPOLIS AND JEBEL TER.]
The geological deposits found in the oasis of Kharga are tabulated on
the following page, commencing with those most recently formed. The
succession, as shown in the table, is that which obtains in
the northern part of the depression, but as far as is known the
same stages occur throughout the oasis, and do not vary either in
thickness or in lithological characters to any great extent. Over
large areas the lower-lying parts of the oasis-floor are formed of
those beds which we have designated the Surface-water Sandstone,
though in
+------------------------+----------------------------+---------+
| | |Thickness|
| Geological System. | Stage. | in |
| | | Metres. |
+------------------------+----------------------------+---------+
| | | |
| |{Sand-Dunes }| |
|RECENT AND } |{Spring Deposits (modern) }| Very |
|PLEISTOCENE} |{Lacustrine Sands and Clays}| variable|
| |{Calcareous Tufa }| |
| | | |
| | | |
| | | |
|LOWER } Lower Libyan| Plateau Limestone | 115 |
|EOCENE} | | |
| Passage Beds| Esna Shales and Marls | 55 |
| | | |
| |{White Chalk }| 70 |
| {Danian |{Ash-grey Shales }| |
| { |{Exogyra Beds | 30 |
|UPPER { | | |
|CRETACEOUS { |{Phosphate Beds | 70 |
| {Campanian |{Purple Shales | 50 |
| {(Nubian |{Surface-water Sandstone | 45 |
| { Series) |{Impermeable Grey Shales | 75 |
| |{Artesian-water Sandstone | 120 |
| +---------+
| Total | 630 |
+-----------------------------------------------------+---------+
places the still older underlying grey shales are exposed. The
purple or red shales generally form the rising ground towards the
escarpments, at the base of which are usually found the phosphatic
beds, with hard, pronounced bands made up of fish-remains and
phosphatic nodules. Above come the Exogyra Beds, with thick bands of
limestone almost entirely composed of large oyster-shells. Rising
up above these is the generally well-marked cliff of grey shales,
capped by a snow-white chalk of much the same age geologically as
the well-known chalk of the South of England. The summit of the
chalk frequently forms a separate plateau, subsidiary to the high
desert tableland, and separated from it by the cliffs formed of
the massive Eocene limestones.
The total thickness of the exposed strata is about 435 metres, a
figure obtained by actual measurement. Numerous borings show the
thickness of the unexposed underlying Impermeable Grey Shales to
be 75 metres, and the deepest borings yet made have pierced the
still lower Artesian-water Sandstone to a depth of 120 metres,
making a grand total of known deposits of 630 metres, or 2,067
feet. The depth to which the water-bearing sandstone extends is at
present a matter of speculation; the point is of great importance
in connection with the water-supply, though up to the present
no borings of sufficient depth have been made to determine its
thickness, nature, and relation to the underlying igneous rocks.
With the exception of a few isolated bosses of eruptive rock in
the desert to the south of the oasis—indications of the granitic
foundation which probably underlies the entire area—the geological
deposits of the oasis-depression, and of the surrounding escarpments
and plateaux, are entirely of sedimentary origin, that is to say,
they were laid down on the shores and beds of pre-existing seas and
inland lakes. The sand-dunes are, of course, an exception, having
been deposited by the wind on the surface of the land. Although,
geologically speaking, the oldest group of sediments with which we
have to deal belongs to the later chapters of the earth’s history,
many hundreds of thousands of years have elapsed since the sandstones
and shales, now forming and underlying the floor of the oasis,
were accumulated on the bed of a vast inland lake. This sheet of
comparatively fresh water was then invaded by the sea, which held
sway in the region while the whole of the series of sediments,
now exposed in the cliffs of the oasis and some 350 metres in
thickness, were being laid down. In Middle Eocene times the sea
commenced to retreat to the north, and the area under description
became dry land with a continually receding shore-line. Since that
time the forces of denudation have constantly been at work lowering
the general surface of the plateau and excavating those depressions
in which alone at the present day man is able to exist.
The Egyptian oases are deep and extensive depressions or hollows
cut down nearly to sea-level through the generally horizontal rocks
forming the Libyan Desert plateaux, and appear to owe their origin in
great measure to the differential effects of subaërial denudation
acting on rock-masses of varying hardness and composition. The
surface-features or configuration of almost any land which has long
been exposed to the powerful forces of erosion are more or less
intimately dependent on the structure and lithological characters of
the underlying rocks. On relative hardness, more than on anything
else perhaps, depend the ultimate positions of mountains, hills,
and plateaux on the one hand, of valleys, plains, and depressions
on the other. Variation in the original conditions of deposition, at
the time when the rocks now forming the Libyan Desert were laid down
on the floor of the sea, has resulted in a preponderant development
in some areas of soft clayey or sandy rocks (as compared with the
hard limestones), and subsequent earth-movements have raised these
beds more in some districts than in others. The result has been
that wherever, during the gradual denudation to which the country
since its elevation has been subjected, these soft deposits have
become exposed on the surface, weathering has proceeded at a greatly
increased rate, and eventually produced deep and broad depressions
separated by high limestone tablelands.
[Illustration: SKETCH MAP
SHOWING THE GEOLOGY
AND THE ANCIENT LAKES OF THE
OASIS OF KHARGA]
But for the presence of comparatively soft formations such as the
Esna Shales, the Exogyra Beds, and the Nubian Sandstone, coupled with
the facts that they have an unequal development in different areas,
and occur at a greater elevation in some localities than in others,
the great depressions of the Libyan Desert would not have come into
existence, or at any rate would have been of comparatively little
importance in the configuration of the country.
The oases are true depressions, completely or partially surrounded by
high escarpments. The oasis of Baharia, for instance, is on all sides
hemmed in by cliffs; on the other hand, Dakhla and Kharga are open to
the south, but as the ground in that direction rises considerably,
they, too, cannot be regarded as other than true depressions. We
have no definite grounds for considering that the erosion of these
depressions can have been the work of previously existing rivers,
and there is no evidence to warrant us in assuming them to have
been formed by local subsidence of portions of the earth’s crust.
What, then, were the agents of denudation and transportation
which operated in the formation of these great depressions? Under
the existing arid conditions the surface rocks, unprotected by
vegetation, are rapidly disintegrated or weathered as the result
of the great diurnal variations of temperature to which they are
subjected (insolation). The weathered material, however, does not
accumulate and form a protective soil-cap, but is carried away by
the wind (deflation), the heavier siliceous grains effecting an
immense amount of abrasion of the exposed rock-surfaces over which
they are swept. Changes of temperature, sand, and wind are, indeed,
the chief agents of erosion and transportation at the present day,
and, given a sufficiency of time and a continuance of favourable
conditions, we can confidently admit the combination to be capable
of effecting a vast amount of earth-sculpture. But the formation,
in this way, of huge hollows 300 to 400 metres deep, and the
removal of material amounting to hundreds of cubic kilometres,
would necessitate the assumption that the present rigorous desert
conditions have obtained for a very considerable period.
Taking all the available evidence of which we are cognizant into
consideration, we do not feel justified in assuming this to have
been the case, especially when we recollect the frequent presence
on the escarpments of thick deposits of calcareous tufa, which
it is evident must have been laid down after the depression had
been carved out to a considerable depth. These tufas are almost
certainly of Pleistocene age, though whether they date from the
early or late part of that period has not been determined. In some
localities they occur as thick, horizontally-stratified beds, and
were evidently deposited on the bottoms of lakes; in other places
they appear as fan-like cakes spread over the face of the cliff,
and may have been formed by springs situated near the summits of
the escarpments. The tufas frequently contain large numbers of
fresh-water shells and an abundance of fossil vegetation, and,
from the presence of casts of the leaves of such trees as the oak,
one is led to refer the deposit to the more humid period which
preceded the incoming of the modern desert conditions.
Although the evidence met with in the field is altogether against
the idea that portions of the plateau have been bodily let down
by subsidence, there are good reasons for believing that tectonic
movements have played an important part in deciding the general
shape of the oases-depressions. For instance, there is considerable
parallelism between the general trend of the Baharia depression
and the folds which pass through that region. As a result of those
folds, it is not improbable that the hard limestone beds were to
some extent broken up, and the soft underlying clays and sandstones
raised as compared with their position on either side of the folded
belt. In Kharga, similarly, the main axis of the depression is, as
we shall show, distinctly parallel to the great north and south line
of flexure, and there is little doubt that a close connection exists
between the two. On the other hand, I know of no folding in the case
of Farafra, which appears to owe its existence solely to the fact
that there was in that region an unusual development of shales at
the base of the Eocene nummulitic limestones. Of the four Egyptian
oases, perhaps Dakhla is the one most easily accounted for, as this
depression may be regarded as simply due to the general northerly
dip of the sedimentary formations, and the gradual weathering back
(northwards) of the great argillaceous series (Exogyra Beds) capped
by the White Chalk. The original limits of the latter may, indeed,
never have been very far to the south.[3]
While all sedimentary strata—such as the limestones, sandstones,
clays, and shales with which we are now dealing—were originally
deposited either quite horizontally or inclined at only a very low
angle, it by no means follows that this horizontality is maintained
when the strata are elevated into dry land. Over the Libyan Desert
as a whole the successive sedimentary formations dip steadily
northwards, but at a very small inclination. This results in every
stage having a wide outcrop, so much so that, if it were not for
the cliff-sections of the Nile Valley and the oases-depressions,
we should have to travel immense distances to obtain any idea of
the true succession of rocks. This general horizontality of strata
appears at first sight to be well maintained in the oasis of Kharga,
as whether we stand on the summit of the eastern escarpment, on top
of the great central hill-massif of Jebel Tarif, or on the plateau
above Ain Amûr, we see everywhere horizontally-disposed beds of
limestone forming the plateaux and upper portions of the cliffs,
with parallel bands of sandstone, shale, and chalk outcropping on
the slopes below.
A closer examination, however, will show that there is in reality
a difference in level of more than 200 metres between the same beds
on either side of the oasis; for the beds capping Jebel Tarif belong
to the White Chalk of the Cretaceous system, and are therefore very
much older than those of Eocene age forming the eastern plateau
(see map and section). This great difference in vertical position
is due partly to a steady dip from west to east, partly to a
remarkable longitudinal flexure running north and south through
the centre of the depression, and partly to a gentle fold near the
base of the eastern escarpment. Along the actual line of flexure,
which passes through Jebel Têr, Jebel Tarwan, Nadûra, Gorn el
Gennâh, and Gertuma (S.S.E. of Bulaq), the different rock-stages
are folded and fractured to a remarkable degree. Throughout the
greater part of its course the flexure approximates to the type of
disturbance known as a simple monocline, but in places, as in Jebel
Têr, it passes into a syncline bounded by nearly vertical faults;
while in others the beds are bent into almost symmetrical basins or
centroclinal folds, typical examples of these structures being met
with at points 6 kilometres south of Kharga village and 2 kilometres
south-west of Qasr Zaiyan.
The importance of this line of folding and faulting must not be lost
sight of, as although the dislocations produced are only actually
visible in the case of the exposed upper beds of the oasis sequence,
the earth-movements to which it owes its origin have had similar
disturbing effects on the underlying and hidden water-bearing
strata. Ball reported that the most striking evidence of faulting
was between Jebel Têr and Jebel Tarif, and showed the fault as
running for a short distance in a N.N.E. and S.S.W. direction, but,
as already mentioned, the line of disturbance is coincident with
the longer axis of the former range, so that the majority of the
wells are on the west or upthrow side of the fault. The effects of
this faulting and folding on the underground water-supply will be
further alluded to in a later chapter.
In the early summer of this year (1908) I followed the line of
flexure southwards in order to determine whether it continued
throughout the oasis. As far as the small eminence of Gala, about
10 kilometres south of Bulaq, it ran in an almost straight line,
but south of that point its course took a distinct bend to the
west, so that the fold was very soon lost in the great belt of
sand-dunes. Beyond this point its continuation could, however, be
inferred by occasional exposures of steeply inclined sandstones,
the most southerly point to which it was actually traced being
in latitude 24° 55′ N., about 15 kilometres S.S.W. of Ain Girm
Meshîm.
[Illustration: GEOLOGICAL SECTION ACROSS THE OASIS FROM JEBEL TARIF
TO THE EASTERN ESCARPMENT.]
Before concluding our remarks on the geology of the oasis we must not
omit to call attention to the beautiful and varied fossil remains
which are almost everywhere to be met with in the calcareous beds
of the hills and escarpments. It is, of course, by the study and
comparison of these organic remains that geologists are enabled to
determine the relative ages of the beds in which they occur, and
thus to correlate them with the rocks of other countries. The lower
argillaceous and arenaceous deposits of the oasis are comparatively
unfossiliferous.
From any of the points of vantage, such as are afforded by the
higher hills within the depression, the general level of the
floor of the oasis does not appear to vary to any great extent,
but actual levelling shows that this is not in reality the case;
and it is this variation of absolute level which is the primary
cause of the very varying volumes of water yielded by the artesian
wells in the different districts.
The average height of the centre of the depression in the
neighbourhood of the village of Kharga is approximately the same
as that of the Nile Valley plain in the latitude of Farshut.
Ball, by comparison of a series of aneroid barometer readings with
the barometric records for the same period at the Cairo observatory,
deduced the value of a point near Kharga village as 86 metres above
sea-level, and used this as his datum in calculating the levels of
other parts of the oasis. Previous aneroid determinations of the
same point had been made by Cailliaud (104 and 118 metres) and by
Jordan (68 metres). But even when the greatest possible precautions
are exercised, aneroid determinations, especially when made with
a single instrument, are necessarily unreliable, and still more
so when used for calculating the levels of different points on a
plain having only comparatively slight irregularities of surface.
Utilizing the figure obtained by the railway surveyors for a point
near the termination of the line, we get values of 58 and 60 metres
above sea-level for Kharga village and Bore No. 1 at headquarters
respectively, and a bench-mark at the latter place, having a value
of 60·1, is used as the datum from which all the heights given
in this book are calculated. Unfortunately it is not possible,
owing to the lack of sufficient check-levels, to state the limit
of probable error, and it must therefore be understood that the
value of our datum, which in the meantime may be accepted as the
best obtainable, is subject to future revision.
From this central point (Bore No. 1) lines of levels have recently
been carried in every direction by Mr. F. E. Apted and myself,
with the result that it has been shown that the general level of
the floor of the oasis rises steadily to the north and falls to
the south. These levels have in all cases been checked, and may,
using the datum mentioned, be accepted as fairly reliable, the
closing errors on the different loops being generally within a very
few centimetres. The altitudes of a few reference points in each
district are given here.
+-----------------------+-------------------------+-----------+
| District. | Point. | Altitude. |
+-----------------------+-------------------------+-----------+
| Headquarters | Bore No. 1 | 60·1 |
| ” | Bore No. 44 | 53·1 |
| Kharga | Bir Ain el Gôs | 70·2 |
| ” | Ain el Sabbagh | 56·9 |
| ” | Ain Zaaf | 87·7 |
| ” | Temple of Hibis (floor) | 75·0 |
| Meheriq | Bir Qattara | 56·6 |
| ” | Ain Mahmud | 64·8 |
| ” | Ain el Burg | 69·6 |
| ” | Ain el Qasr | 76·0 |
| ” | Ain el Ghazâl | 84·6 |
| South of headquarters | Ain Harrân | 43·5 |
| ” ” ” | Ain Ali Morad | 33·0 |
| ” ” ” | Ain Bellal | 28·4 |
| ” ” ” | Ain el Tawîl | 19·7 |
| Gennâh | Ain Estakherab | 71·3 |
| ” | Ain Magarin | 74·6 |
| ” | Ain el Ghuâta (north) | 48·2 |
| ” | Ain Zaiyan | 41·6 |
| ” | Qasr Zaiyan | 20·7 |
| El Dêr | Bore No. 22 | 92·3 |
+-----------------------+-------------------------+-----------+
The Government Survey maps show a portion of the oasis floor as
lying below sea-level, the difference of height between the Kharga
village datum and a point just south of Qasr Zaiyan being given as
104 metres. Detailed surveying shows that this estimate is excessive,
the true difference being about 37 metres only. Although no actual
reading has yet been obtained below sea-level—the lowest being
+2·6 metres at a point 3½ kilometres north-east of the northern
end of the Gorn el Gennâh, or nearly midway between that hill and
Ain el Tawîl—it is evident that in this district the floor is
only very slightly higher than the sea, and it may be that at one
or two points its level is actually lower.
South of Qasr Zaiyan no revision of previous levels has as yet
been made. According to Ball’s figures, the village of Beris is
approximately 10 metres lower than Kharga.
While discussing the subject of levels it may be useful to note the
relative heights of the escarpments and hills within the oasis. The
edge of the eastern plateau varies from 350 to 400 metres above
sea-level, while the plateau to the north of Um el Dabâdib has a
general level of about 400 metres. Jebel Tarif appears to be very
slightly higher, while the highest peak on Jebel Têr is not much
more than 300 metres. The altitudes of these points with reference
to the village were mostly determined by Ball by trigonometric
observations with an eight-inch theodolite, and can therefore be
relied on as being accurate.