IV.—PHYSICAL TRANSFORMATION OF ROCKS.
The geological features of Egypt as presented to-day are the results of the formation through varying conditions and the subsequent deformation of the rocks composing its solid crust; we may next consider the varied agencies through whose action these are now undergoing transformation. A comparatively small portion of this country is undergoing erosion by the sea, and if elevation be still taking place, there is rather gain from the sea than destruction by it.
Very different, however, are the meteorological agencies which are at work fashioning the land as a whole, the effects of wind-blown sand, rain, and river being of prime importance. Different in these respects are the Eastern and Western Deserts of Egypt. If the western border of the river be examined in Southern Egypt, and especially in Nubia, the sight of huge masses of golden-tinted sand filling every wind-sheltered hollow might well leave the impression that the vast desert plain behind was covered by a pall of sand. Closer study has shown that these wind-swept expanses afford no protection or resting-place for the finer sands and that consequently their floor must be formed of the more solid materials which wind cannot carry before it. If this be realized, no astonishment will be felt that the Libyan Desert surface is composed of limestone, or of coarse gravels from between whose larger fragments all the finer sand has been swept away.
Fig. 5.—Sand-erosion of Sandstone Cliff at Gebel el Tunb. Wadi Qena, Eastern Desert.
Fig. 6.—Amphitheatre in Side-valley of Um Leseifa, north-east of Qena, due to erosion of Limestone by the action of Temporary Torrents.
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However, the wind-borne sand leaves its mark on the limestones, which in some places are seamed by delicate grooves parallel to the sand-blast, and in others, where they are softer, have been sculptured into low hummocks often scattered over immense tracts of country. The sand itself has a strangely local distribution, advancing across the desert in lines of enormous length, and usually trending in an almost meridional direction. The supply does not come from the south or south-west, as might at first sight be expected, but from the north, and most of the great dune-systems, which occur around and beyond the great oases, have their termination in the southern direction. Even the series of dunes, over 100 metres high, which prevented the Rohlfs party advancing westward from Dakhla to Kufra, come to an end further to the south, leaving the wide sandstone plains bare.[6] In the oasis of Kharga the dominant longitudinal type is replaced by huge crescentic dunes which, separated by broad spaces clear of sand, follow one another along a north-south line, and are not stopped in their onward march even by ridges of considerable size. The reasons have still to be found for these lines of special sand aggregation, though when studying the cataract district of Amara, the initial formation of a dune-system was seen to be determined by a local depression, which had given sufficient protection for the formation of a sandy base on which the dune could then be built up.
Denudation Effects in a District of Sedimentary Rocks.