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.