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?