III.—DEFORMATION OF ROCKS.
Examination of the rock exposures at many localities in the neighbourhood of Cairo bears witness to the fact that the strata of limestone which were laid down horizontally upon the bed of the sea are no longer in the position they once occupied, but are now inclined to the horizontal plane at angles which are easily perceptible. This is especially noticeable in the two shallow cuttings under the Great Pyramid, where the strata are inclined 5 degrees to the south, revealing the reason why the whole Pyramid slope descends so rapidly in the southward direction. Again, if standing at the base of the same mighty structure, the gaze be allowed to wander over the broad expanse of the Nile Valley to the scarp of the Moqattam hills behind the Citadel, it will be seen that the white limestone of their lower slopes forms, not a long horizontal wall, but an arch, sloping strongly both north and south of the Citadel. There has been obviously change of form, but how has it taken place?
We learn with surprise that an earthquake shock in distant San Francisco or in the inhospitable regions of Turkestan has recorded itself through the delicately-poised recorder at Helwan, long before the dire news of destruction has flashed along the telegraph wire to the same destination. There is a realization of the instability of the earth’s crust, in spite of the solidarity of its component parts, but though the sudden shocks bring home this truth, it is not so readily grasped that day by day and hour by hour parts of the earth’s crust are slowly rising and others sinking, offering stern problems to the dwellers on the shores where these changes are most markedly taking place. On the eastern shores of England, towns and forests are being submerged beneath the relentless advance of the sea; on the coasts of the Scandinavian peninsula, etc., on the other hand, beaches formerly beneath the sea now stand high above the influence of its waves, and in Egypt coral-reefs which once grew beneath the waters of the Red Sea rise in places to over two hundred metres above it in bold hills or steep-sided terraces. Though possibly of far greater importance and significance than the sudden convulsions which have left so deep an impression on the mind of man, these movements are nevertheless so imperceptible that they arouse little attention.
A second type of deformation is noted where heavy masses of one rock rest on other and softer materials. The underlying beds are then often compressed and contorted; clays, for instance, are drawn out into thinner laminæ, giving rise to shales, while the massive rocks above either slip on the surface of the lower ones, if these be impermeable, and so permit of a water-layer forming along the junction, or else descend by sheer weight, producing a confused area of mixed materials in front of the still unshattered cliffs. Examples of this nature abound in Egypt wherever the Eocene limestones rest on the Cretaceous clays; on the railway from Armant to Matana the resulting effects are well observed near Shagab. Here in the main cliff the massive limestones are seen resting on the soft bluish clays, while in front is a wilderness of low hills in which limestones and clays are mixed, broken and contorted. But while these deformations may be irregular and local, there remain greater pressure-effects which have been regional in character, and by whose agency the solid rocks have been folded in the most remarkable manner, strata once horizontal being thrown into arches, or anticlines, and basin-like curves, or synclines. The importance of these changes cannot be over-estimated, and some of the marked features of Egyptian scenery depend directly on these effects. Reference has already been made to the slope of the Pyramid plateau, but the character of Egypt in far broader outline depends on the results of these pressures and the foldings so produced. A glance at the map suffices to show that many of the salient features in this country present a remarkable similarity and parallelism. The eastern cliff-wall of Kharga Oasis preserves a notable parallelism to a portion of the Nile Valley, though the latter be separated by many kilometres of wild desert plateau from the oasis; another portion of the Nile Valley also agrees with the Gulf of Suez in the broad outlines of its trend.
Fig. 1.—Two Anticlines and a Syncline.[3]
A section taken from Baharia or Kharga Oasis to the shores of the Red Sea reveals the fact that Egypt proper is bounded on the west by a low flat arch which has brought the underlying sandstones nearer to the surface, giving rise to the great oases.[4] These are mainly present in the sandstone areas, and are in part bounded by cliff-walls composed in many instances of clays at the base and limestones at the summit. To the east, on the other hand, rise the Red Sea hills, the central core of a steeply inclined arch in which the resistant granites and schists now rise high above the low-lying sandstone country which flanks them. Between these two arches is the flat-bedded syncline in which the nummulitic limestone is the conspicuous member, the strata in central Egypt having in consequence a half cup-shaped form, of which one half, the northern, may have disappeared by fracture beneath the waters of the Mediterranean Sea. This cup-like structure may be due to Egypt having not only been folded in a north-west and south-east direction but also almost at right angles, this latter folding giving rise to such remarkable features as the Wadi Araba, the Qena bend, and possibly affording the fundamental explanation for the great S-shaped bends of the Nile. In other and more mountainous regions, such as the Alps and Himalayas, these solid earth-waves may be under such immense pressure that their crests begin first to turn over like those of an advancing wave of the sea, and then may be broken in such a way that mighty masses of strata are rent asunder, those portions which are uppermost being thrust for great distances over the underlying beds. These extreme folds ending in fracture, or overthrusts, are as yet but little known in Egypt, though Dr. Ball has reported an interesting case from the neighbourhood of Abu Harba, and some of the phenomena of dislocation observed near the borders of the Gulf of Suez may possibly be explained as resulting from movements of this nature.
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.