CHAPTER XI.. * * * * *

* * * * * =TECTONICS AND GENERAL GEOLOGY.= * * * * *

=Evidence of a former Pluvial Period.=

Given a sufficiently long period for their activity, the denuding and transporting agencies at work at the present day are capable of accounting for most of the superficial sculpturing of South-Eastern Egypt. The country is not absolutely rainless, and within a decade most of the dry valleys have been for a few hours the beds of streams, the result of rain storms. There is practically no frost in this part of the world, so that disintegration by the freezing of water in crevices of the rock does not occur on any large scale; the diurnal variations of temperature, are, however, so great that this cause alone is very potent in breaking up rock material. The disintegrated matter accumulates as heaps of debris and sand, ready to be transported towards the Nile or the sea by the streams which follow the next rainfall. Both in erosion and in the transport of sand, wind is a very active agent, and accounts for the formation and distribution of immense quantities of sand. Thus the mountains are slowly being lowered, and the rocky valleys between them are being widened and deepened, even at the present day, and the accumulations of sand on the coast-plain and elsewhere are being slowly increased in thickness.

But when we look at the great wadis, often hundreds of kilometres in length, cut to a depth of fifty metres with a width of half a kilometre through the sandstone plateaux which separate the mountain ranges from the Nile, it is difficult to conceive that rainfall and denudation have not in the past been greater than at present. In our own day, it is but seldom that the great wadis convey streams as far as the Nile or the sea, their waters being usually absorbed by the sandy bed before the end is reached; erosion nowadays is practically confined to the upper reaches of the wadis, and unless we postulate greater rainfall in the past, inconceivable ages must have been occupied in the erosion of these great channels. We are thus driven to believe that what is now a very dry area was formerly one of considerable rainfall. This belief is supported by the traces of glaciation in Europe, for it is natural to infer that when temperate Europe had an arctic climate, northern Africa had a temperate one; the effect, whatever its cause, being practically equivalent to an increase of latitude. This change of climate is equally evidenced by geological observation in other parts of Egypt. It is even likely that the climate of Egypt may be slowly changing at present; but the change within the historical period has been so small as to be practically negligible.

=Origin of the Red Sea.=

If the 200-metre contour of the bottom of the Red Sea, shown on Plate I, be examined, it will be found to exhibit great indentations towards the great mountain masses, while there is a curious projection including the Island of Zeberged which mimics the present Ras Benas. Some of the indentations of the contour line lie in the direct prolongation of existing great wadis, such as those of Lahami, Khoda, Hodein, Di-ib, and Serimtai. The obvious suggestion from this coincidence is that the sea has encroached on the land since the drainage-system had substantially its present form, and we infer a sinking of the region at no very remote geological epoch. The central parts of the Red Sea attain depths of over 2,000 metres; thus this sea was a great and deep one even when the level of its waters, relative to the land, was 200 metres lower than now. We have no information which would give us a clue to the origin of this primitive sea, but the inference from the contours is that the present extent of the Red Sea has been caused by a great general subsidence of the land, and not by trough-faulting as has hitherto been usually stated.

The subsidence just referred to was even greater than would be gathered from a consideration of the present coast-line. At intervals along the entire eastern coast of Egypt are hills of gypsum; these are never found except close to the present sea-borders, and the natural deduction is that the gypsum beds were deposited when the sea was at a higher level than at present. At Ras Benas, the gypseous strata reach altitudes of nearly 200 metres, so that at the time when the gypsum was formed the Red Sea must have covered a much greater area than now, extending in fact approximately to the contour of 200 metres above present sea-level. As to the epoch when this greater extent of the sea existed, we should have a clue if we knew the age of the gypsum beds, which unfortunately is not the case; but they are almost certainly younger Tertiary beds, possibly Miocene or even Pliocene, so that in any case the Red Sea is a depression of considerable antiquity.

=Possible Former Extent of the Eocene Rocks.=

Eocene rocks are entirely absent from the district, and the same is the case with Cretaceous rocks younger than the Nubian sandstone. But if we go westward along the parallel of Berenice, across the Nile into longitude 23° 30′, we come to the plateau face of Gebel Garra, where there are exposed thicknesses of about ninety metres of Eocene limestone and 240 metres of Cretaceous marls. These beds cover great expanses further west and north; they evidently once extended beyond their present limits, and we may ask whether they ever reached over the Red Sea mountains here, as is the case in North-Eastern Egypt. To this question no answer can be given; from Berenice to Gebel Garra is a distance of over 300 kilometres, and even a very gradual thinning of the beds eastward would account for their absence from the main mountains; at the same time the denudation which has removed every trace of the hard Eocene limestone from the plain between Gebel Garra and the Nile may well have done the same further east.

=Original Extent of the Nubian Sandstone.=

The Nubian sandstone is found on both sides of the watershed ranges, and the question raised by its distribution is in regard to its possible original continuity. Did the Nubian sandstone once extend over the present igneous mountains, or was it laid down on either side of a great island ridge? In other words, did the Red Sea mountains exist as such in Cretaceous times, or have they been subsequently elevated, and the Nubian sandstones which covered them denuded away? To this question it may be stated at once that no certain answer can be given; but a good deal of study has been devoted to the facts bearing upon it, and these facts will be briefly enumerated and discussed below.

Evidence from Volcanic Intrusions.—That the Nubian sandstone is younger than the igneous rocks in general is proved by the almost complete absence of intrusions into the sandstone. Apart from an interbedded diabase sheet in the north part of the region and a small basic dyke near Gebel Awamtib, the igneous rocks appear nowhere to penetrate the Nubian beds. The sandstone is a fortiori younger than the metamorphic rocks into which the igneous masses have been intruded.

Evidence from Present Distribution.—Coming now to the distribution of the sandstone, though we find it on both sides of the main ranges, there is not a trace of sandstone among the higher mountains; this of course proves nothing, for denudation would be most active among the peaks and in districts of maximum up-thrust, and sandstone is an easily erodible rock. The long tongue of sandstone plateau which terminates in Gebel Anfeib extends indeed right across the main watershed, but a reference to the orographical map (Plate I) will show that this extension is along a north-west to south-east general depression, where the Wadis Hodein and Garara form a cut across the map separating the main mountain masses into north and south groups. The presence of the sandstone tongue here would be equally well accounted for whether the mountains were elevated before or after the deposition of the Nubian beds.

Not only is the sandstone absent from the main mountains, but it is never found in such close proximity to them as to render a decisive answer to our question. Thus the eastern scarp of the sandstone plateau in the north part of the area overlooks a broad tract of low country between it and the mountains; and similarly from Gebel Anfeib one overlooks low country to the north, east, and south. On the eastern side of the mountains the sandstone deposits are restricted to small patches considerably removed from the main summits, and even in most cases from their foot-hills.

Evidences from Structural Features.—With regard to the structural evidences to be gathered from the sandstone itself, it was hoped that the observations of the dip of the beds at different places would throw light on the question as to whether their deposition antedated the mountain formation or no.

To the north of latitude 24°, the eastern edge of the sandstone scarp showed beds differing but little from the horizontal, though very gentle folding in various directions is probable.

Further south, near Gebel Zergat Naam, much more decided evidences of folding, and even of dislocation, were met with. The head of the Wadi el Kreim, south-west of Zergat Naam (see the geological map on Plate XX) is probably a line of fault, for here on the south-west of the wadi we have sandstones coming right down to the wadi floor, with a dip of 30° to the south-west near the edge, becoming flatter the further we go from the wadi, while on the other side are crushed and brecciated schists. Going further west, in the sandstone hill-mass which lies twelve kilometres west of Zergat Naam, the dip of the beds is in the opposite direction, being 10° to 15° north-east, and granite appears at the foot of the steep west-south-west slope; this, with the preceding observation, seems to indicate a synclinal fold terminated near Zergat Naam by a fault up-thrusting the schists and syenite.

Where the Wadi Garara cuts through the sandstone hills to receive the Wadi el Kreim, the beds dip markedly to the south, the observed inclinations being 60° or more at the north edges, rapidly falling to 20° or less further south; the north faces show granite and schists at their base.

Further west, on the way to Gebel Um Harba, the sandstone of the hills showed dips to the east of 15°. At Gebel Um Harba itself there are thick beds of sandstone dipping 13° east-north-east, while all around the mountain one looks out over beds having approximately the same inclination.

At Gebel Um Khafur, the dip is 13° to 14° to the north-north-east, and is very constant over a large area. From the north side, where the plain is 380 metres above sea-level, the hills rise with a succession of dip slopes and basset-edges over a horizontal distance of 2·9 kilometres (measured perpendicular to the strike) to the triangulation beacon at 560 metres above sea, the beds all along dipping at 13° or 14°. Unless there is step-faulting here along the strike-wadis (see Fig. 60) the total thickness of Nubian sandstone here is over 450 metres. This is a much greater thickness than has been noted anywhere else in Egypt, and I am inclined to think that there is step-faulting along at least two of the strike wadis which separate the hill-mass into ridges.

In approaching Gebel Awamtib from the north-west, I crossed over a small patch of diorite in the sandstone at the pass from Wadi Um Terbi into Wadi Awamtib, and a basic dyke was found cutting the sandstone of a spur of Gebel Awamtib. The beds of Awamtib itself dip pretty uniformly a little north of west.

About nine kilometres to the south of Gebel Awamtib, a station was taken on a sandstone headland with schists and quartz veins at its floor, and afterwards I skirted the limit between sandstone and granitic rocks on the way to Gebel Um Reit. All along this route there was no suggestion of sharp folding or faulting; the beds were nearly horizontal right up to the limit, where the granite hills rise suddenly (see Fig. 61).

From the top of Gebel Um Reit, which itself is granite, the sandstone limit could be seen to the north and east; the beds dip 30° in places; in those on the east the direction of dip is about south. The region round Um Reit is evidently one of considerable disturbance, but the sandstone is too far off for one to get any precise idea of its nature from the summit.

Going northwards from Um Reit across the Wadi Saalek and up one of its branches into the head of Wadi Muegil, there is a good exposure of faulted sandstone overlying schists just before reaching the pass. Here (see Fig. 62) one of the faults is a distinct overthrust, with a north-easterly strike, and there are other faults in a parallel direction.

From the Wadi Muegil northwards past the Galt el Aguz to Gebel Um Harba there are gentle dips and curvings of the sandstone beds, with a predominant dip about north-east. At the Galt el Aguz the sandstone rocks are much tumbled about, but this appears to be due to fall of over-hanging beds.

At the pass from Wadi Um Arta into the head of Wadi Silsila, the dip of the sandstone is north-north-east. In its lower part, Wadi Silsila passes between sandstone ridges the dip of which is constantly east-north-east.

Round Bir Abraq and the triangulation station on Gebel Abraq (see large scale map on Plate XV) the main direction of dip is east-south-east; the pool of Bir Abraq itself is under projecting slabs of sandstone dipping in this direction. Bir el Sunta, a little further north, appears to lie in a syncline, the beds of the hills to the north dipping south-east, while those to the south dip north-west.

Approaching the sandstone from the east by Wadi Hodein, one traverses granite country with felsite dykes and enters suddenly into high sandstone plateaux. The beds at the Abu Saafa Springs dip about 5° northwards.

The beds of Gebel Dif and Gebel Anfeib have a prevalent dip on the west side to the east and north-east, while on the east face the dip appears to be in the reverse direction. This great mass of sandstone plateau may thus be a syncline with a north-westerly strike (i.e., a strike roughly parallel to the main direction of the watershed mountain ranges), but the eastern face was not examined in detail, and it may be that the whole mass dips to the north-east; some outlying small sandstone masses near the head of Wadi Edunqul show no reversal, the dip being still east-north-east. At Bir Dif the sandstones are variously tilted, and probably faulted.

The sandstones on the east side of the watershed ranges, near Gebel Ranga, were observed by Dr. Hume to dip towards the sea. In the south part of the region there are some low hills forming two series of ridges between the sea and Gebel Kolaiqo, separated by hills of red aplitic granite; these hills were not actually visited, but were mapped from some little distance, and the dip of the beds was not measured, but the nature of the rocks was confirmed by specimens of sandstone brought back by guides sent to the place.

Summing up the dips and disturbances of the sandstone beds at the different places, it is clear that the strata are the more disturbed, the more closely they approach the main mountain ranges; this and the seaward dip of the beds near El Ranga are so far in favour of the view that the sandstone may once have extended right over the present mountains. But it will be seen that the disturbances of bedding are of an irregular character, especially between Gebels Zergat Naam and Um Reit, where the folding and faulting are often in directions quite distinct from that of the main mountain axes, and it may be that these disturbances are due to later movements rather than to the elevation of the main mountain ranges. So far as observations on the sandstone itself go, therefore, the question as to whether the Nubian sandstones of the Red Sea border ever directly joined those on the west of the mountains remains open; the observed disturbances of the beds give us clear proof that considerable earth movements have taken place since the sandstone was deposited, but they are inconclusive as to whether these same movements caused the elevation of the great mountain ranges.

=The Igneous and Metamorphic Rocks.=

Turning now to the main mountain-forming rocks, we find them composed of the two great classes, igneous and metamorphic. Though in places we have transition members, such as granites passing into gneiss, yet on the whole the two groups are quite distinct, and we find typical igneous masses rising in the midst of equally typical schists. Under these circumstances there can be no hesitation in considering the two groups to be of different ages, and that the igneous rocks must be the younger.

The igneous masses are divisible geologically into the three classes of (a) lavas, (b) dykes, and (c) plutonic rocks.

Lavas.—Amongst the igneous rocks, the lavas are those most poorly represented. Apart from the andesite of Gebel Sufra, and the diabase sheets in or under the Nubian sandstone, there are only a few occurrences of volcanic rocks in all the area, a circumstance which is easily explained by the enormous denudation which has gone on and the fact that lavas, being superficial out-pourings, will have been most exposed to denuding forces.

Dykes.—Dykes seam the schists and igneous rocks, frequently in such numbers and with such parallelism as to give to the land the form of a succession of ridges separated by long narrow depressions, forming “dyke country.” In nature, the dykes vary from extremely acid rocks like aplites and quartz-felsites, to very basic forms such as diabase and basalt. There is on the whole a preponderance of basic over acid types. As already remarked, only one instance of a dyke cutting the Nubian sandstone has been observed in the district. Another significant feature in the distribution of dykes is that while dykes of all kinds are found cutting schists and acid plutonic igneous rocks, the basic plutonic masses are as a rule free from dykes; this suggests that the basic plutonic rocks may be on the whole younger than the acid forms.

Plutonic Rocks.—The plutonic rocks include granites, syenites, diorites, gabbros, pyroxenites, amphibolites, and peridotites. But these are not all of equal importance, and a natural division from the field observations is a two-fold one into acid and basic groups, which are tolerably well marked off from each other and are most likely of different ages. The acid class on this view consists of the granites and syenites, while the basic class contains all the other rocks above-mentioned. The granites are on the whole of a very acid type; syenite is very scarce and nearly always occurs in close association with granite. Of the basic group, probably the most abundant and most typical rock is gabbro; diorites and the various ultra-basic rocks are almost always closely associated with gabbro. Where acid and basic rocks occur in proximity (as for instance near Bir Abraq, where an acid granite occurs side by side with a serpentine) there is a sharp change, without transitional forms.

The much greater abundance of dykes in the plutonic rocks of the acid group as compared with those of basic composition, as mentioned above, inclines us to regard the basic group as the younger—a view which is further supported by the greater frequency with which a tendency to gneissose structure is noticeable in the acid rocks. The fact that the basic members are frequently in a higher state of decomposition than the acid ones is not contrary to this view, being due to the greater ease with which their constituents undergo weathering; and as explained on p. 315, the fissured state of the altered peridotes is probably not due to the same earth forces which have sheared the granites, but to internal stresses set up by the expansion of the rocks on serpentinisation.

Though we may be fairly sure that the basic plutonic rocks are on the whole younger than the acid ones, we have no certain guide from which to estimate the geological age of either. We know from their relations to the sandstone that they are both older than the Upper Cretaceous, but we cannot say how much older; they may be anything from Archæan to Jurassic; the circumstance that similar rocks underlie Carboniferous strata in Sinai inclines us to place them at least as far back as Palæozoic times.

Gneisses and Schists.—The gneisses and schists which cover so much of the country are obviously older than the plutonic masses which are intruded in them, and for these metamorphic masses we need have small hesitation in speculating on an Archæan age. They exhibit a wonderful variety of composition. Most of the gneisses are doubtless sheared ancient igneous rocks, such as granite and diorite, and a similar origin may be assigned to many of the schists. Other rocks, as for instance the clay-schists, graphite-schists, and marble, have almost certainly originated from the metamorphism of ancient sedimentary rocks. But in a large number of cases we have at present no clue to the parent rock from which the schists have been formed.

=Summary of Geological History of South-Eastern Egypt.=

Having in the foregoing pages discussed the evidences for the relative ages of the different classes of rocks and their mutual relations, we may now endeavour to reconstruct the past geological history of this part of Egypt from the information gathered. In this process we shall reverse the order of consideration taken above, and begin with the oldest rocks.

The schists and gneisses probably represent, not the original crust of consolidation of the earth, but a complex of ancient sedimentary and igneous rocks, laid down in pre-Palæozoic times and subsequently crushed, folded, and faulted into mountains which were subsequently denuded and worn down. In parts of the main mountain-masses we may possibly still have the cores of some of these ancient elevations, but most of the present mountain peaks are formed of a later series of igneous rocks.

The first igneous intrusions into the schists and gneisses were the granites which form such peaks as Gebels Nugrus, Faraid, and Elba. Then followed crushing and folding of these rocks, giving them often a gneissose structure and opening fissures, up which came later intrusions, mostly of basic rocks, in the form of dykes. With the dykes or later came great basic intrusions of gabbroid rocks, forming such masses as Gebels Dahanib, Gerf, and Meisah.

A long interval now followed of which the rocks contain no record. We do not know whether the area was submerged or not in Palæozoic and early Mesozoic times; but if it was, all trace of the deposits of these ages has vanished in the great denudation which surely took place before the Upper Cretaceous sea swept over the country and deposited the Nubian sandstone. We do not know whether the Red Sea mountains then stood up as islands, or whether they were subsequently elevated. Nor do we know whether the area remained wholly or partly submerged during the Eocene period. But we are sure that at some time between the Upper Cretaceous and Oligocene epochs there was a great elevation of the land, with folding and faulting, especially in the areas now occupied by the great mountain masses, and possibly the mountains had their origin in this movement of elevation. The Red Sea depression may well have originated in a complementary sinking at this same period. The depth of the sea is of the same order (2,000 metres) as the height of the mountains on the adjacent land.

In the succeeding Oligocene period the land was being sculptured into something like its present form, and probably the main drainage lines of to-day were then formed.

About the Miocene epoch there was a sinking of the crust in this region. The Red Sea increased in area, and then, probably as the results of evaporation in a closed sea, deposits of gypsum were laid down. A subsequent elevation in Pliocene or post-Pliocene times raised these deposits along the present shores.

In geologically recent times a further gentle elevation has gone on, giving rise to slightly raised coral-reefs and sea-beaches. During the glacial period of Europe, the rainfall in Egypt was probably greater than at present, and during this period the great wadis received almost their final sculpturing. After the change from this rainy climate to the dry one of to-day, erosion still went on, though more slowly, in the hills; but on the plains and along the coast accumulation of sands took place, partly owing to wind transport, and partly owing to the insufficiency of the drainage waters to carry their load as far as the Nile or sea. The abundance of coral-reefs in the Red Sea is largely conditioned by the lack of in-flowing streams of silt-laden water; for the coral animal flourishes only where the water is clear.

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