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.[138]
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.