The principles and objects of geology, with special reference to the geology of Egypt

by W. F. Hume

Fig. 7.—Um Leseifa camp (looking west). The harder beds of limestone form precipitous ledges.

Denudation Effects in a District of Igneous Rocks.

Fig. 8.—View of Diorite Hills near Gebel Sobeir, Eastern Desert of Egypt.

[Facing p. 11.]

Not only are limestones grooved and seamed, but certain areas of the Western Desert are covered with curious melon-shaped masses, harder concretionary portions remaining after the softer materials of the beds have been carried away by wind-action, whereas in the region to the east of the Nile these concretions are still enclosed in the softer limestones.

Still more striking is the effect of the wind-blown sand in the sandstone and granitic regions. Here the complex composition of the granite has made it a ready victim. The softer felspars and mica having been worn by the impact, leave the quartz grains loose upon the surface, and give the rock a “frittered” appearance. Holes have been formed in the windward side of the blocks, and the sand contained in them clearly shows the agency to which they owe their origin.

Near the eastern edge of the Western Desert the effects of water-action become more conspicuous, on the borders of the Fayûm terrace-formation being a marked feature, and there is a transition to the dominant characteristics which mark the limestone country eastward of the Nile. Here, deep and intricate valley-systems have been cut out from a plateau which a few kilometres from the Nile is as flat as the great western plains. One of the best known examples is the Wadi Hof, near Helwan, with its ramifications, terminating in steep cliff-faces having all the appearances of “dry waterfalls.” Any doubt as to the active agent in their production is set at rest by an examination of the excellent photographs taken by various observers during the great storms which almost annually burst over this region. (The lecture was illustrated by a series of slides showing Wadi Hof in flood, and the cascades descending the “waterfalls” which terminate its side-channels, these being most kindly lent to the writer by Herr Züst, of the Electrical Service, Ministry of Public Works). The great annual and diurnal temperature variations (over 50° C.) aid in the work of denudation by preparing an immense amount of broken material through the contraction and expansion which they produce. As every material expands and contracts according as it is heated or cooled, so the different component parts of the rocks composing the earth’s crust are in constant movement with regard to one another, and the less homogeneous they are the greater the effect in breaking them up into small masses or particles. These loosened fragments which cover the surface of the desert are thus ready to be swept away by the rain-waters, and as we have already seen, it is owing to these effects, superadded to more subtle changes next to be considered, that the old volcanic rocks of Abyssinia yield the rich silt or mud of the Nile Valley. It would be difficult to estimate the rapidity with which these wild ravines are being deepened by any comparison with water-wearing effects in Europe. Any beds of soft sands and clays are rapidly dissected by the torrent waters, a feature which readily explains the absence of conspicuous hills in the Eastern Desert east of Esna, where the Cretaceous clays form the dominant constituent in the geological structure of the country. Whatever the effects of sand-erosion in the Western Desert or rain-erosion in the hills and on the plateaus of the Eastern Desert of Egypt, they come relatively but little under the notice of the dweller on the Nile, to whom the river-erosion and the reformation of new materials become of primary importance.

Even the powerful agency of frost cannot be entirely dismissed from consideration in Egypt. Owing to the expansion of water when converted into ice, the rocks in whose cracks the water has collected are split asunder, and as we have recently noted, temperatures lower than 2° C. have been recorded in Cairo during the present winter (1910). On the great desert plateau which extends from Kharga Oasis to the Nile Valley, temperatures of 24° F. and 30° F. were also observed, and in the Red Sea hills and Sinai frost must be of common occurrence, as one mountain in the latter peninsula was ascended in snow, and the higher peaks are frequently covered in a white pall.