CHAPTER VIII.. * * * * *

* * * * * =GEOLOGY.—SEDIMENTARY ROCKS.= * * * * *

Only a small proportion of South-Eastern Egypt is covered by sedimentary rocks. As will be seen from a glance at the geological map on Plate XX, sedimentary strata are to be found only in the north-western and central portions of the area, where the Nubian sandstone which covers so much of the country close east of the Nile terminates in a long south-easterly projection, and along certain portions of the coast, where small outliers of Nubian sandstone and patches and strips of gypseous limestones and coral reef occur. The oldest rocks of undoubted sedimentary origin, the Nubian sandstones, are of Cretaceous age. Tertiary beds are absent, except for the gypseous strata of Ras Benas, which may possibly be Miocene. Quaternary deposits are represented by Pleistocene and Recent coral reefs and beach deposits, and by accumulations of alluvial downwash, blown sand, and calcareous tufa.

The sedimentary deposits may thus be classified into:—

(1) Recent Deposits (coral reefs and raised beaches, blown sand, alluvial deposits, and calcareous tufa).

(2) Gypsum and Gypseous Limestones (of uncertain age, possibly Miocene).

(3) Nubian Sandstone (Cretaceous).

=Coral Reefs and Raised Beaches.=

Coral reefs fringe almost the entire coast-line of South-Eastern Egypt, rendering the coast one of the foulest in the world for shipping. Coral forms the islands of Wadi Gemal, Gulhan, Mukawar, Mirear, Seyal, and Halaib, these rising to a few metres above the sea; but in general it is not met with on the main land except close to the shore. The reefs may extend, slightly raised, some distance inland, but cannot be seen owing to the sand and downwash from the mountains which covers the sloping coast-plain. At low water great expanses of nearly level coral reef, stretching out for a kilometre or more from the shore, are exposed in places, especially near Ras Benas. Frequently a line of reefs can be seen running for many miles parallel to the coast at a distance of a kilometre or more from it; these outer reefs can be traced in windy weather by the breaking of waves on them, and in calm weather by the lighter colour of the water over them. There is frequently deep water between the outer reefs and the shore. Openings in the outer reefs, through which boats can pass into the sheltered water within, are called mersas by the Arab boatmen; they frequently occur opposite the mouths of the larger wadis. Coral reef forms a tough, hard, and very porous limestone, with an extremely rough surface on which the structure of the astraean and other corals composing it can be clearly seen. It has been used for building the jetty at Halaib, and rough blocks of it were anciently employed for building at Berenice, as well as in the tombs at Suakin el Qadim; but its porosity and intractability would never recommend it as a building stone if any other could be found locally.

In close association with coral reefs are beach deposits of Pleistocene age. These occur round the hills near the tip and on the north side of the peninsula of Ras Benas, where they cover the slopes of the gypseous limestone hills. Low hills and banks of very white aspect occur near the sea further north; these are probably also raised beaches, though they have not been visited and may possibly be gypsum. Near the tip of Ras Benas these beach deposits are gritty limestones with abundant casts of lamellibranchs and gasteropods, mostly in bad preservation, as well as various echinoids, among which M. Fourtau has identified the species Brissus carinatus as one of the most abundant.

=Blown Sand.=

The dune-forming sand in South-Eastern Egypt is exactly similar to that which covers such large areas in the Libyan Desert. It consists of well-rounded quartz grains, averaging about a millimetre in diameter; the grains are usually of a golden-yellow colour, owing to their being coated with a thin superficial film of iron oxide.

The only area where sand dunes are of any considerable magnitude or extent in South-Eastern Egypt is the tract called I Hubâl (lit. “the sand dunes”) which extends from Wadi Meisah to Wadi Aideib, fronting the hills of Ti Keferiai, Geror, Balatitda, Sul Hamid, and El Sela. The north faces of all these hills and the smaller hills north of them are swathed in thickly accumulated sand to such an extent as to make travelling over them difficult. The reason for the heavy sand accumulation at this particular place is that the north-west wind, which is the prevalent wind over Egypt proper, here meets with south-easterly winds coming up the Red Sea, causing a local calm and a consequent dropping of the sand borne from the Nubian sandstone areas by the north-west wind. Thus the same climatic conditions which give rise to the rich vegetation on the Elba mountains cause at the same time extreme desert conditions in the tract immediately north-west of them.

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 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.

It is to the alluvial material in the wadis that the Eastern Desert owes most of its perennial water supplies and the vegetation it possesses. The alluvial deposits are the great conservers of water. Pools form, it is true, in the bare rocky beds of the higher drainage lines, and may last for many months where they are screened by the walls of a gorge from wind and sun, and are in consequence not subject to rapid evaporation. But such pools are difficult of access, and afford but a precarious source of water supply because they are liable to dry up if a long period passes without rain. The alluvium of the wadis absorbs the rainfall and protects it from evaporation, so that even in very dry years water may be found by excavating in it at suitable places to depths of a few metres. Almost all the wells which are so important to travellers crossing the desert from the Nile are of this character. The abundance of trees which flourish in so many of the wadis likewise derive their nourishment from the water conserved in the alluvium of the wadi floor.

=Calcareous Tufa.=

Small deposits of calcareous tufa of recent origin have been noted in the Wadi Um Tundeba and in a gorge on the east side of Gebel Ghuel, as well as round the little trickling spring called Megwel Hamida in the south part of the region.

At Um Tundeba the deposit occurs in a little gully close to a well known galt or pool of rain water. The deposit is not extensive, and has doubtless been formed by the evaporation of trickling drainage-water which had absorbed lime from the rocks. The tufa (10,374) is a pale brown rock of rather porous nature; it envelops fragments of schist-debris from the surrounding rocks.

The calcareous tufa of Gebel Ghuel is a more impressive deposit, though its total mass is not very great. It occurs at a point of sudden fall in a narrow rocky gorge leading to the Wadi Ghadir in about latitude 22° 53′. Proceeding up the gully one is confronted with a great curtain-like mass of tufa covering the face of a high ledge like a solidified cascade. Here also the origin is clearly due to trickling of lime-laden drainage-waters over the ledge of rock, which forms a step in the wadi floor.

Fragments of tufa were also seen round about Gebel Allawi, though the rock was not traced to its source. As only a relatively small number of rocky gullies have been explored, it is likely that similar deposits to those above described occur in many other places.

=Gypsum and Gypseous Limestones.=

The occurrences of gypsum and gypseous limestone beds in South-Eastern Egypt are restricted to the coastal regions. The most prominent of the deposits are those on Ras Benas (see map on Plate XXI), where the beds form white hills rising to 188 metres above sea-level. Further north, gypseous deposits have been found by Dr. Hume to exist near the sea at Bir el Ranga, and by Mr. Ferrar the same beds have been recorded as occurring near the coast in the neighbourhood of Wadi Igli. It appears probable that the gypseous strata form a continuous or nearly continuous strip extending along the coast down to latitude 24° 22′, as shown on the geological map on Plate XX, but more complete observations may show that the distribution of the beds is somewhat different from that indicated. On the coast-plain south of Ras Benas, gypseous beds are not exposed except in the extreme south-east corner of Egypt, where they form small patches at Halaib and round the wells of Ti Kureitra.

At Ras Benas, where the beds have been studied in most detail, they consist of gypsum and anhydrite [11,513] alternating with sandy marls and marly sands, forming hills much cut-up by steep sided narrow ravines. The weathered faces of the rocks are very soft, and the disintegrated material forms a stretch of soft gypseous sand, into which one’s feet sink four or five centimetres at every step, between the hills and the shore. At Halaib, gypseous limestones crop out from under the gravel of the coast-plain and form low banks; the gypsum is here associated with calcareous grits [12,152, 12,114] and conglomerates, the latter having boulders of igneous rock set in a calcareous matrix. Near Ti Kureitra wells, the gypsum exposures contain much crystalline selenite, blocks of this material being used in the masonry lining of the wells. Both at Bir el Ranga and on Ras Benas the gypsum is found to contain small pockets of native sulphur.

No fossils have been found in the gypseous strata, and their geological age is uncertain. The beds are younger than the Nubian sandstone, since they overlie that formation at Bir el Ranga. Their occurrence only near the coast leads one to regard them as having been formed after the Red Sea occupied its present position; but whether they originated as direct gypseous deposits, or were produced by the alteration of pre-existent Cretaceous or Tertiary limestones is not yet quite certain. It is noteworthy that at Ras Benas, where the gypseous strata rest on diorite and hornblende granite, the igneous rocks are considerably altered as if by weathering, with a strong brick-red colour due to the oxidation of ferruginous matter.

PLATE XXI.

Ball. Geography & Geology of South-Eastern Egypt.

Photo-Metal-Process. Survey Dept. Cairo 1910. (60-190)]

=The Nubian Sandstone.=

The Nubian sandstone covers a comparatively small portion of South-Eastern Egypt, occupying only about one-tenth of the total area under consideration. The greater portion of it forms the eastern termination of the vast sandstone plateaux which stretch eastward from the Nile towards the igneous and metamorphic back-bone of the Red Sea mountains, but there are in addition some small patches on the eastern side of the watershed which are interesting as showing that the deposition of the sandstone was not confined to the western side of the mountain ranges.

The most northerly exposure of Nubian sandstone in the district here described extends with some breaks southward from near Gebel Sufra to near Gebel Homr Akarim, forming low hills. Further south, it comes in again on both sides of the Wadi Garara, forming the hills of Felieiti and Abu Hashim. In the sandy plain around the Wadi Timsah are outlying sandstone hills of considerable height and extent, the principal being Gebels Nuggur, Mulgata, and Ziraga. Further south-west, the Nubian sandstone forms an extensive high broken plateau stretching southward from near Gebel Zergat Naam to near Gebel Um Reit, sending out a long tongue south-eastwards across Wadi Dif to near Gebel Nigrub el Tahtani. This tract of broken sandstone plateaux bears various names in different parts, Gebels Um Harba, Um Khafur, Dagalai, Shebakhit, Awamtib, Um Sididad, Abraq, Hodein, Dif, Anfeib, and Kala, all forming parts of the same great sandstone mass, though separated by wadis from each other. Small outliers of Nubian sandstone cap the hills of Gebels Reietit, just north of latitude 23°, near the meridian of 34°, and Gebel Seiga, in latitude 22° 44′, longitude 34° 16′.

On the eastern side of the watershed, Nubian sandstone extends for some distance near the coast opposite the Gulhan islands, and also forms small thin outliers south of the Wadi Hodein. The most southerly point where the sandstone has been met with in the area is a small outlier in latitude 22° 36′, a few kilometres east of Gebel Hamra Dom.

In petrographical characters the Nubian sandstone is remarkably uniform, consisting of medium-sized silica grains set in a more or less ferruginous cement; the colour varies from nearly white, through various shades of brown, to nearly black, according to the amount of iron present. It is generally well bedded, and frequently much jointed. There is usually a marked absence of the clay beds which are associated with the Nubian sandstone in other parts of Egypt. The basal beds are generally pebbly, forming conglomerates in some places. Concretions are found in the rock at some points, notably to the north of Gebel Um Harba. Some of the concretions are hollow, the shell being of hard dark ferruginous sandstone while the interior is filled with a powder of snow-white chalky matter containing sand grains. The only fossils observed in the sandstone within the area described were collected by Mr. Charteris Stewart in the plain of Um Harba; they consist of crocodile scales and various shells. From these and the fossil shells found further west in sinking a well in Wadi Abu Rahal, it may be concluded that the Nubian sandstone here, as in the Nile Valley, is of Cretaceous age.

Intrusions of igneous rock into the sandstone have only been observed at two points in the area, viz., near the western foot of Gebel Awamtib, where a basic dyke runs through the sandstone, and on the Red Sea coast, near Bir el Ranga, where Dr. Hume has found andesite interbedded in the sandstone. A bed of diabase, now much altered, underlies the sandstone near Wadi Muelih, and is possibly of contemporaneous origin.

In thickness, the Nubian sandstone attains a maximum of about 350 metres in several places round about Bir Abraq and the Wadi Hodein. Its thickness must at one time have exceeded this, for the upper surfaces have suffered much denudation, and are not capped by younger rocks.

Some interest attaches to the maximum altitude reached by the Nubian sandstone beds. The highest point at which it has been observed is at Gebel Seiga, whose sandstone cap is 905 metres above sea-level. The highest point reached by the rock on the great plateau round about Wadi Hodein is Gebel Kala, 846 metres above sea.

In contradistinction to the same beds which form the plateau between longitude 34° and the Nile, the Nubian sandstones within the area here described show considerable disturbances from their original horizontal bedding, the dips being as a rule greater and more variable the nearer one approaches to the watershed mountain-ranges. These tectonic disturbances, which are important in connexion with the geological history of the region, will be considered in Chapter XI.

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