Part 6
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]