Chapter III: Part 3

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.

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:

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.