Summary of Geological History of South-Eastern Egypt.
Having in the foregoing pages discussed the evidences for the relative ages of the different classes of rocks and their mutual relations, we may now endeavour to reconstruct the past geological history of this part of Egypt from the information gathered. In this process we shall reverse the order of consideration taken above, and begin with the oldest rocks.
The schists and gneisses probably represent, not the original crust of consolidation of the earth, but a complex of ancient sedimentary and igneous rocks, laid down in pre-Palæozoic times and subsequently crushed, folded, and faulted into mountains which were subsequently denuded and worn down. In parts of the main mountain-masses we may possibly still have the cores of some of these ancient elevations, but most of the present mountain peaks are formed of a later series of igneous rocks.
The first igneous intrusions into the schists and gneisses were the granites which form such peaks as Gebels Nugrus, Faraid, and Elba. Then followed crushing and folding of these rocks, giving them often a gneissose structure and opening fissures, up which came later intrusions, mostly of basic rocks, in the form of dykes. With the dykes or later came great basic intrusions of gabbroid rocks, forming such masses as Gebels Dahanib, Gerf, and Meisah.
A long interval now followed of which the rocks contain no record. We do not know whether the area was submerged or not in Palæozoic and early Mesozoic times; but if it was, all trace of the deposits of these ages has vanished in the great denudation which surely took place before the Upper Cretaceous sea swept over the country and deposited the Nubian sandstone. We do not know whether the Red Sea mountains then stood up as islands, or whether they were subsequently elevated. Nor do we know whether the area remained wholly or partly submerged during the Eocene period. But we are sure that at some time between the Upper Cretaceous and Oligocene epochs there was a great elevation of the land, with folding and faulting, especially in the areas now occupied by the great mountain masses, and possibly the mountains had their origin in this movement of elevation. The Red Sea depression may well have originated in a complementary sinking at this same period. The depth of the sea is of the same order (2,000 metres) as the height of the mountains on the adjacent land.
In the succeeding Oligocene period the land was being sculptured into something like its present form, and probably the main drainage lines of to-day were then formed.
About the Miocene epoch there was a sinking of the crust in this region. The Red Sea increased in area, and then, probably as the results of evaporation in a closed sea, deposits of gypsum were laid down. A subsequent elevation in Pliocene or post-Pliocene times raised these deposits along the present shores.
In geologically recent times a further gentle elevation has gone on, giving rise to slightly raised coral-reefs and sea-beaches. During the glacial period of Europe, the rainfall in Egypt was probably greater than at present, and during this period the great wadis received almost their final sculpturing. After the change from this rainy climate to the dry one of to-day, erosion still went on, though more slowly, in the hills; but on the plains and along the coast accumulation of sands took place, partly owing to wind transport, and partly owing to the insufficiency of the drainage waters to carry their load as far as the Nile or sea. The abundance of coral-reefs in the Red Sea is largely conditioned by the lack of in-flowing streams of silt-laden water; for the coral animal flourishes only where the water is clear.
[138]Suess, in The Face of the Earth (English edition, Vol. I, p. 374), states that “the Red Sea itself is, however, a trough-subsidence, probably the greatest in the world.”