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.
[Facing p. 10.]
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.
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.
The nature of a river system need not be dealt with in much detail here, as I have already discussed this subject in “Survey Notes” for April, 1907, under the title of “River Characteristics as illustrated by the Nile.” It may be well, however, to recall that a normal river passes through three distinct phases of activity. In its mountain tract (for most large rivers arise in the higher altitudes) there is maximum erosion and backward growth of the river system. In its central portion, or valley tract, the stream is acting as a transporter of eroded material, and such erosion as there is, is downward rather than sideward. Finally, the plain tract is the region of deposition of the materials so carried, erosion being lateral, and the growth of the stream bed forward in the form of a fan-shaped delta where the transported sands and clays enter the sea.
But this general succession may be further complicated by circumstances depending upon the geological conditions. In Egypt and the Sudan the Nile passes from areas where it flows peacefully and quietly, usually of considerable breadth and bounded by fertile lands, to others in which it is restricted, dashing down steep slopes in rapids and cataracts. A geological examination has shown that in the first case the river is flowing over and between sedimentary homogeneous rocks, such as the limestones and sandstones, while in the second instance it has entered regions composed of heterogeneous igneous and metamorphic rocks, such as the granites, gneisses and schists. The production of these rapids is due to the combination of steep slope and the difference between harder and softer materials, the rapidly-moving waters wearing away those more easily denuded, while the compact members remain as obstacles to their advance, and are only slowly worn away along joint-planes and other lines of weakness. In the Third Cataract, hard bars of granite rising through softer gneiss at right angles to the river course have produced the main rapids; elsewhere, as at the Bab el Kebir, near Wadi Halfa, the river has taken advantage of a thin dyke of soft rock traversing an extremely hard diorite, so that the stream has worn a narrow gully between steep rock-walls, where the intensity of the rush of water is greatly exaggerated owing to its being restrained and fettered by the narrowness of the passage. In some cases the same result has been produced owing to the existence of a line of fracture, or fault, across the stream, the waters taking advantage of this line of least resistance. The general erosion in these rapids is accompanied by great local effects where eddies and whirlpools are produced, and the sand and rocky fragments act as abrading agents. Pot-holes are formed in the solid rock, and rapidly deepened by the intense effects of this nature produced during times of flood, the result being splendidly illustrated in some of the smaller islands of the First Cataract at Aswan.
A river is, in fact, the main agent combining the effects of transformation and reformation, new strata being produced in its plain tract as the result of the eroding activities in its upper reaches. Much of the detrital material is also carried seaward to form deposits of marine sands and muds along the shore-lines of the continents, these themselves becoming, should subsequent differential movement of land and sea take place, the sandstones and clays of future continental areas.
But there are other agencies at work as transformers on and within the earth’s crust. There are in most rocks a series of divisional planes, which may be either vertical or inclined, and to which the name of joints has been given. These may arise from various causes. Both in sedimentary and igneous rocks they are in part due to contraction during consolidation—in the former when they lose their contained water, in the latter when they solidify from a molten condition. Joints may also be called into being by the effects of internal pressures and movements within the earth’s crust, such structures having been experimentally reproduced by Daubrée in materials under stress by torsion and by simple pressure. The granite of Aswan displays such jointing to a marked degree, giving rise to remarkable hills composed of huge boulders of granite piled on one another.