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.
Fig. 60.—Section of Sandstone at Gebel Um Khafur.
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.
Fig. 61.—Sketch section of junction of sandstone with granite, west of Gebel Um Keit.
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.
Fig. 62.—Sketch of faulting near Wadi Saalek.
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.