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of underground water which is there withdrawn by the river is probably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The length of the tract over which the warm water was being withdrawn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]