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.