Sand accumulations of less magnitude occur in and about the lower reaches of the Wadi Hodein, in the great valley to the north of Berenice, on the plain west of Gebel Um Harba, and at some few other places, but they are seldom so extensive as to cause a serious hindrance to travel.
Besides transporting the ordinary dune-forming sand, the action of the wind is responsible for a considerable amount of redistribution of the coarser granitic and other sand produced by the disintegration of igneous rocks. In high winds this coarse sand, in angular grains as large as peas, is blown about on the coast-plain to considerable distances from the places where it was originally deposited by water transport.
Alluvial Deposits.
The degradation of the mountain and hill masses which is constantly going on under sub-aerial agencies (disintegration by variations of temperature, and transport by water during occasional rain storms) has in the course of centuries resulted in immense accumulations of alluvial material (sand and gravel) along all the drainage lines and on the plains. Disintegration is in many places greatly facilitated by the crushed nature of the rocks, due to the tectonic movements which they have suffered during the process of folding and mountain formation. This fact is specially evident in the case of the more basic rocks. Serpentines, for instance, are frequently so crushed and cracked that one may search in vain over a whole mountain for a place where the rock is sufficiently free from cracks to allow of a hand-specimen of the ordinary size being extracted without breaking up into fragments under the hammer. In the more acid rocks, like the granites, crushing has been a less powerful aid to disintegration, and here the diurnal temperature-range has been the principal factor; the rock, being an aggregate of crystals of different materials with different coefficients of expansion, has been easily broken up at its surface by the constant differential motion of its component grains, resulting in the formation of coarse granitic sand. Chemical action in denudation is of quite subordinate importance. Though some chemical action has taken place, as for instance in the kaolinisation of the felspars, we find as a rule that the alluvial accumulations consist principally of unaltered rock fragments.
In material, the alluvial accumulations are of the same varied nature as the mountains from whose degradation they have originated, ranging from very basic mineral substances, such as serpentine, to the most acid, such as quartz. The relative proportions of the different materials in the alluvial deposits do not, however, correspond as a rule with the proportions in which they enter into the mountain masses. Not only are heterogeneous rocks, such as granites and diorites, far more rapidly disintegrated by temperature-changes than are the more homogeneous rocks such as felsites and serpentines, but the fragments produced by the disintegration of granitic rocks are rounder and of much smaller size than the fragments resulting from the breaking up of felsites, schists, and serpentines. The result is that granitic detritus is transported to greater distances and distributed over larger areas than the detritus of the closer-grained and more homogeneous rocks. In approaching a mountain composed of about equal parts of, say, granite and serpentine, we accordingly find the lower portions of the wadi consist chiefly of granitic sand, while the coarser and more angular detritus of serpentine is only seen in abundance in the higher parts of the drainage channel. The nature of the alluvium of the wadi floor occasionally gives its name to the wadi; thus, of the two wadis which by their junction form Wadi Khoda, one is called Wadi Salib el Azrak, because its alluvium consists of dark rock fragments, while the other is called Wadi Salib el Abiad, because its floor is formed of white granitic sand. (Azrak = dark; Abiad = white).
In size, the fragments composing alluvial detritus vary from huge blocks weighing tons down to the finest sand whose grains are only a fraction of a millimetre in diameter. As a rule, of course, the larger fragments are found near the heads of wadis, and the finer sands in their lower reaches and on the plains. But often we find great boulders mixed with the sand at immense distances from the parent mountain mass; the heavier rain storms which occasionally break over the mountains produce for short periods such immense rushes of water down the wadis, that great boulders are swept along and incorporated in the finer material which is deposited in times of more normal rainfall. Thus it is no uncommon thing in sinking a well in a wadi fifty kilometres or more from the mountains to encounter great boulders in the sandy alluvium.
In shape, the alluvial fragments vary very much, according to their mineral nature and the amount of rolling they have received in transport. Foliated rocks, like schists and slates, produce typically flaky fragments which seldom attain any high degree of rounding; the same is true of serpentines which have been shattered by crushing. Granitic rocks produce generally more symmetrical fragments, though granitic sand is typically angular unless it has been transported for considerable distances. The larger blocks are generally rounded in the case of granitic rocks, even when they have not travelled for any great distance; for if a cubical block of granite be exposed to weathering, disintegration takes place most rapidly at the corners, and the block eventually becomes more or less spheroidal without having moved at all. This can be well seen in such masses as Gebel Selaia, where the granite boss is thoroughly well rounded in situ. The same phenomenon can be noticed in certain dolerites, where portions of the rock detached by joint planes have weathered in situ into forms like cannon balls. The more homogeneous rocks like felsite and serpentine, as well as the foliated rocks like gneisses, schists, and slates, do not show much of this rounding in situ, as they yield to the stresses of expansion and contraction by breaking along planes of crushing or foliation, producing separate masses which preserve most of their angular shape. In the lower reaches of the wadis and on the plains, of course, all the materials are more or less rounded, owing to the attrition to which they have been subjected during transport.
Of the thickness of the alluvial sands and gravels very little is known. But that it exceeds eight metres in many of the wadis is proved by wells sunk to that depth. On the coast-plain it may be vastly thicker, for Grabham[122] mentions that borings made in the maritime plain near Port Sudan have been carried down to 1,000 feet (305 metres) in the deposits. Only a very small proportion of the alluvial detritus brought down the wadis enters either the sea or the Nile; for the storm-formed streams, though violent in the mountainous tracts, commonly cease by being absorbed in the thirsty wadi floors and plains before reaching either the sea or the river. Bearing this in mind, and the ages through which the actions of denudation and transport have gone on, we can well imagine how great must be the total quantity of detritus accumulated.
In some wadis there are high banks of old detritus through which the streams have cut their way. In the Wadi Meneiga, for instance, there are terraces of alluvium, some eight or ten metres high, on either side of the present water channel; these terraces, which are about two kilometres lower down the wadi than the wells, are covered with rude rubble ruins, the remains of dwellings erected out of reach of the occasional streams which rush down the wadi after rain.
At the east foot of Gebel Hamra Dom are some well stratified friable sand rocks and sandy clays, about ten metres thick, ending abruptly against the granites and schists of the hill-mass. These beds appear to be ancient alluvial deposits. They contain ferruginous scales and show sun-cracks in places, while the materials are finer and more distinctly stratified than one would expect to be brought down from the mountain under present conditions.