The total thickness of the exposed strata is about 435 metres, a figure obtained by actual measurement. Numerous borings show the thickness of the unexposed underlying Impermeable Grey Shales to be 75 metres, and the deepest borings yet made have pierced the still lower Artesian-water Sandstone to a depth of 120 metres, making a grand total of known deposits of 630 metres, or 2,067 feet. The depth to which the water-bearing sandstone extends is at present a matter of speculation; the point is of great importance in connection with the water-supply, though up to the present no borings of sufficient depth have been made to determine its thickness, nature, and relation to the underlying igneous rocks.

With the exception of a few isolated bosses of eruptive rock in the desert to the south of the oasis—indications of the granitic foundation which probably underlies the entire area—the geological deposits of the oasis-depression, and of the surrounding escarpments and plateaux, are entirely of sedimentary origin, that is to say, they were laid down on the shores and beds of pre-existing seas and inland lakes. The sand-dunes are, of course, an exception, having been deposited by the wind on the surface of the land. Although, geologically speaking, the oldest group of sediments with which we have to deal belongs to the later chapters of the earth’s history, many hundreds of thousands of years have elapsed since the sandstones and shales, now forming and underlying the floor of the oasis, were accumulated on the bed of a vast inland lake. This sheet of comparatively fresh water was then invaded by the sea, which held sway in the region while the whole of the series of sediments, now exposed in the cliffs of the oasis and some 350 metres in thickness, were being laid down. In Middle Eocene times the sea commenced to retreat to the north, and the area under description became dry land with a continually receding shore-line. Since that time the forces of denudation have constantly been at work lowering the general surface of the plateau and excavating those depressions in which alone at the present day man is able to exist.

The Egyptian oases are deep and extensive depressions or hollows cut down nearly to sea-level through the generally horizontal rocks forming the Libyan Desert plateaux, and appear to owe their origin in great measure to the differential effects of subaërial denudation acting on rock-masses of varying hardness and composition. The surface-features or configuration of almost any land which has long been exposed to the powerful forces of erosion are more or less intimately dependent on the structure and lithological characters of the underlying rocks. On relative hardness, more than on anything else perhaps, depend the ultimate positions of mountains, hills, and plateaux on the one hand, of valleys, plains, and depressions on the other. Variation in the original conditions of deposition, at the time when the rocks now forming the Libyan Desert were laid down on the floor of the sea, has resulted in a preponderant development in some areas of soft clayey or sandy rocks (as compared with the hard limestones), and subsequent earth-movements have raised these beds more in some districts than in others. The result has been that wherever, during the gradual denudation to which the country since its elevation has been subjected, these soft deposits have become exposed on the surface, weathering has proceeded at a greatly increased rate, and eventually produced deep and broad depressions separated by high limestone tablelands.

[(Large-size)]

SKETCH MAP
SHOWING THE GEOLOGY
AND THE ANCIENT LAKES OF THE
OASIS OF KHARGA

But for the presence of comparatively soft formations such as the Esna Shales, the Exogyra Beds, and the Nubian Sandstone, coupled with the facts that they have an unequal development in different areas, and occur at a greater elevation in some localities than in others, the great depressions of the Libyan Desert would not have come into existence, or at any rate would have been of comparatively little importance in the configuration of the country.

The oases are true depressions, completely or partially surrounded by high escarpments. The oasis of Baharia, for instance, is on all sides hemmed in by cliffs; on the other hand, Dakhla and Kharga are open to the south, but as the ground in that direction rises considerably, they, too, cannot be regarded as other than true depressions. We have no definite grounds for considering that the erosion of these depressions can have been the work of previously existing rivers, and there is no evidence to warrant us in assuming them to have been formed by local subsidence of portions of the earth’s crust.

What, then, were the agents of denudation and transportation which operated in the formation of these great depressions? Under the existing arid conditions the surface rocks, unprotected by vegetation, are rapidly disintegrated or weathered as the result of the great diurnal variations of temperature to which they are subjected (insolation). The weathered material, however, does not accumulate and form a protective soil-cap, but is carried away by the wind (deflation), the heavier siliceous grains effecting an immense amount of abrasion of the exposed rock-surfaces over which they are swept. Changes of temperature, sand, and wind are, indeed, the chief agents of erosion and transportation at the present day, and, given a sufficiency of time and a continuance of favourable conditions, we can confidently admit the combination to be capable of effecting a vast amount of earth-sculpture. But the formation, in this way, of huge hollows 300 to 400 metres deep, and the removal of material amounting to hundreds of cubic kilometres, would necessitate the assumption that the present rigorous desert conditions have obtained for a very considerable period.

Taking all the available evidence of which we are cognizant into consideration, we do not feel justified in assuming this to have been the case, especially when we recollect the frequent presence on the escarpments of thick deposits of calcareous tufa, which it is evident must have been laid down after the depression had been carved out to a considerable depth. These tufas are almost certainly of Pleistocene age, though whether they date from the early or late part of that period has not been determined. In some localities they occur as thick, horizontally-stratified beds, and were evidently deposited on the bottoms of lakes; in other places they appear as fan-like cakes spread over the face of the cliff, and may have been formed by springs situated near the summits of the escarpments. The tufas frequently contain large numbers of fresh-water shells and an abundance of fossil vegetation, and, from the presence of casts of the leaves of such trees as the oak, one is led to refer the deposit to the more humid period which preceded the incoming of the modern desert conditions.