Subsequently, subsidence taking place, the Eocene sea submerged the area, and deposits were laid down on the uneven Cretaceous land in an unconformable and overlapping manner. In Baharia the lowest member of the Eocene of Egypt, the Esna Shales, is not present, although further to the south towards Farafra it has been observed. On the east side, some sandstones and clays met with below the white chalk beds of the outer plateau may belong to this division.
The first undoubted Eocene deposits in the Baharia area are the limestones with Operculina and Nummulites which unconformably overlie different members of the Cretaceous in the north and west sides, and eastward of the south end. The whole of the Eocene deposits are here, however, only a few metres thick, which contrasts strangely with the enormous thickness of the deposits of the same age in the Nile Valley. This is intelligible, however, on the supposition that near the subsiding Cretaceous land the conditions for continued accumulation of deposits were not so favourable as further to the east, where deeper water conditions obtained.
Subsequently, in Post-Eocene times, the whole underwent upheaval, and it is probable that during this elevation the main synclinal fold[63] was produced, together with the minor anticline. The evidence for placing the date of the formation of the syncline anterior to the deposition of the ferruginous grits, limonite, etc. (Series No. 3) stands on the following basis: the absence of proof of the folding in question having affected the beds of Series 3, and the presence of a horizontal deposit of limonite on the upturned edges of the strata, at the point where the fold meets the eastern scarp ([page 66]). About the same time, probably, basalt and dolerite was intruded into the Cenomanian rocks below.
Formation of depression.As a result of the sharp folds the upper limestones were cracked, and their denudation by natural agencies followed, forming a slight hollow similar in shape to that which the oasis now exhibits; the agent of denudation cannot be stated with certainty, but whatever force came into operation it would find easy work in the cracked-up rocks, and still easier would be its task in partly removing the soft Cenomanian sandstones and clays after the harder limestones had disappeared. The primary excavation of the hollow was followed by the formation of a great lake, in which were laid down deposits of sandstone, quartzite, and iron-ore; this lake doubtless surrounded islands, represented to-day by those hills which still preserve their limestone-caps; it extended, or similar lakes existed, beyond the oasis-limits, forming the quartzites and ferruginous sandstones passed on the way from Maghagha to Baharia, and was perhaps continuous with the Oligocene and post-Oligocene sea which covered a large part of the country to the north.
In later times the area finally became continental and denudation gradually sculptured the oasis to its present form; this sculpturing would no doubt proceed rapidly in the moist climate which is known to have existed in Egypt in Pliocene and early Pleistocene times, and is being continued to-day by the powerful agency of the desert wind-borne sand and changes of temperature.
The water-supply of the oasis is probably derived from the tropical rains of the mountainous regions of Central Africa, the water from which penetrates the ground and flows northwards along permeable beds of sandstone, etc., in which it is confined by other impermeable strata, until tapped naturally or artificially in the great oases or depressions of the Libyan Desert.
[35]Voyage à Méroé, etc., op. cit.
[36]Geologie, u. Palæontologie der Libyschen Wüste. Cassel, 1883.
[37]Zittel had divided the Cretaceous of the Western Oases into the following main divisions:—