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diff --git a/77091-0.txt b/77091-0.txt new file mode 100644 index 0000000..e3093bb --- /dev/null +++ b/77091-0.txt @@ -0,0 +1,3054 @@ + +*** START OF THE PROJECT GUTENBERG EBOOK 77091 *** + +[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.] + + + +*** END OF THE PROJECT GUTENBERG EBOOK 77091 *** |