Fig. 2.—Overfolding of Strata.
These intense movements become masked under the influence of the denuding hand of time, the contorted strata may again sink beneath the sea, new beds are laid down horizontally on the upturned edges of the older series and the result is the production of an unconformity between the two members, which differ in age, in inclination, and in fossil character. Sometimes rolled fragments of the older stratum are interposed between it and the new overlying beds, further revealing the activity of denudation before the newer member began to be deposited.
If fracture by overthrust be unknown in Egypt, another type of fracture has produced effects of a far-reaching character. In certain regions of the earth, folding is no longer the conspicuous method by which the rock-components of its crust are displaced with regard to one another. In many instances small fissures have been observed in which the beds on one side have been thrown down to a lower level than they are on the other. These faults are frequently the result of great earthquakes, many striking examples of such occurrences having been noted during the major earth-movements within the last hundred years. It is probable, indeed, that many of the most important of these, such as the destructive earthquake of Messina, are due to further settling of the strata in relation to fracture-lines already determined. Naturally, this implies that there are certain spaces formed deep beneath the earth’s surface, which permit of these efforts to restrengthen the weak spots by filling up the gaps. In the extreme outskirts of Egypt these fault-lines have produced marvellous and striking effects, the most conspicuous being the remarkable depression which, commencing at the Gulf of Aqaba, penetrates far into the continent of Asia, giving rise to the Dead Sea (many hundred metres below the level of the Mediterranean), to the Jordan Valley, to the Lake of Tiberias, and the valley which continues this line northward. I have had the opportunity of personally studying some of these fault-lines in the Sinai peninsula, tracing them from an arched fold in the north, whose sides were being let down by faults in a series of steps, to a trough-fault, in which the younger strata are displaced bodily between the older ones. Where undisturbed, the succession in Sinai shows sandstone lying on granite, and limestone (containing definite groups of fossils) on sandstone. In the valleys due to these trough-faults, the granite hills tower 500 metres on each side, capped by small outlying fragments of the overlying sandstone, whereas in the valley itself the only rocks visible are limestones and sandstones often tilted at high angles, and thus revealing the tremendous displacement to which they have been subjected.
Fig. 3.—Unconformity exhibited in the Fayûm.
Fig 4.—Fault exhibited by a Coal-seam.
Considering the earth-movements of Egypt as a whole, the evidence shows folding to have been the more important type of displacement on its western side, and intense faulting to have been most conspicuous in the east, while between these two extremes, the relative importance of folding and faulting will remain a contested point. The discussion will only close when the borders of the Nile Valley and the Gulf of Suez have been mapped with the accuracy and precision of a Geological Survey in Europe—a pious aspiration, whose realization can scarcely be hoped for while the broad geological picture is still being filled in.
Egypt, then, has passed through a long history, of which the following seems to be the record put in dogmatic form. The ancient schists and granites which form the central core of its eastern arch, or anticline, the Red Sea hills, are witnesses to a period of sedimentation, of volcanic action, and of the influence of deep-seated molten igneous masses. Step by step these were revealed by denudation, becoming part of an ancient continent. In addition to a brief marine invasion during the Carboniferous period, there was a second advance of the sea in Cretaceous times, which is represented first by the deposit of such coarse detrital materials as the sands composing the Nubian sandstones. Then, as the depression increased, only the finer clayey materials reached the Egyptian region, and finally the sea covered, if not the whole, at least the greater part of its area for a lengthened period. With the close of the Eocene epoch a reverse movement set in, by which the pure white marine limestones of the Moqattam hills were capped by the sandy limestones and clays forming the brown-tinted beds of their upper portion, the whole being, in its turn, covered by the coarse flint and quartz gravel which containing silicified trees[5] is the chief desert-former in the immediate neighbourhood of Cairo, as also of a large area in north-west Egypt. Any gain by the sea since this period has been comparatively local and limited, and the present conditions suggest that the land is gaining on the sea rather than the reverse. The great fold-movements which we have seen to determine so many important features are relatively of very late date in this series of events, on their nature and position depending the present character and visible extent of the different formations.