CHAPTER XI.
TECTONICS AND GENERAL GEOLOGY.
Evidence of a former Pluvial Period.
Given a sufficiently long period for their activity, the denuding and transporting agencies at work at the present day are capable of accounting for most of the superficial sculpturing of South-Eastern Egypt. The country is not absolutely rainless, and within a decade most of the dry valleys have been for a few hours the beds of streams, the result of rain storms. There is practically no frost in this part of the world, so that disintegration by the freezing of water in crevices of the rock does not occur on any large scale; the diurnal variations of temperature, are, however, so great that this cause alone is very potent in breaking up rock material. The disintegrated matter accumulates as heaps of debris and sand, ready to be transported towards the Nile or the sea by the streams which follow the next rainfall. Both in erosion and in the transport of sand, wind is a very active agent, and accounts for the formation and distribution of immense quantities of sand. Thus the mountains are slowly being lowered, and the rocky valleys between them are being widened and deepened, even at the present day, and the accumulations of sand on the coast-plain and elsewhere are being slowly increased in thickness.
But when we look at the great wadis, often hundreds of kilometres in length, cut to a depth of fifty metres with a width of half a kilometre through the sandstone plateaux which separate the mountain ranges from the Nile, it is difficult to conceive that rainfall and denudation have not in the past been greater than at present. In our own day, it is but seldom that the great wadis convey streams as far as the Nile or the sea, their waters being usually absorbed by the sandy bed before the end is reached; erosion nowadays is practically confined to the upper reaches of the wadis, and unless we postulate greater rainfall in the past, inconceivable ages must have been occupied in the erosion of these great channels. We are thus driven to believe that what is now a very dry area was formerly one of considerable rainfall. This belief is supported by the traces of glaciation in Europe, for it is natural to infer that when temperate Europe had an arctic climate, northern Africa had a temperate one; the effect, whatever its cause, being practically equivalent to an increase of latitude. This change of climate is equally evidenced by geological observation in other parts of Egypt. It is even likely that the climate of Egypt may be slowly changing at present; but the change within the historical period has been so small as to be practically negligible.
Origin of the Red Sea.
If the 200-metre contour of the bottom of the Red Sea, shown on [Plate I,] be examined, it will be found to exhibit great indentations towards the great mountain masses, while there is a curious projection including the Island of Zeberged which mimics the present Ras Benas. Some of the indentations of the contour line lie in the direct prolongation of existing great wadis, such as those of Lahami, Khoda, Hodein, Di-ib, and Serimtai. The obvious suggestion from this coincidence is that the sea has encroached on the land since the drainage-system had substantially its present form, and we infer a sinking of the region at no very remote geological epoch. The central parts of the Red Sea attain depths of over 2,000 metres; thus this sea was a great and deep one even when the level of its waters, relative to the land, was 200 metres lower than now. We have no information which would give us a clue to the origin of this primitive sea, but the inference from the contours is that the present extent of the Red Sea has been caused by a great general subsidence of the land, and not by trough-faulting as has hitherto been usually stated.[138]