Plate XXXVI
Fig. 79. The maritime plain and Irba Mountains
Yemêna oasis in foreground and gravel-covered ridge across middle distance
Plate XXXVII
Figs. 80 and 81. Corals, in position of growth, on the summit of Jebel Têtâwib
The accumulation of this vast mass of gravel and sand in the manner described has taken a length of time compared to which a human life is but a moment. Even from the geological point of view it has been not inconsiderable. There is abundant evidence that the plain was formed, much as it is now, at a time when the coast-line was entirely different, and though there is no good evidence of the country’s having been other than desert throughout historical time, there probably was a greater rainfall when the formation of the plain was in full swing.
The sandstone hills are particularly interesting in that one finds a regular layer of coral on their summits, which shews that they were once nearly level with the sea, and are in fact coral reefs which have been elevated to heights of from 100 to 1000 feet. In some of those hills to which I have had access the corals on the summit are wonderfully well preserved, and by this fact, and that the species are indistinguishable from those now living in the sea, prove the elevation of the hills to have been geologically recent. Further, the larger coral colonies are at once seen to be still in the position in which they grew, not tilted or overthrown in any way. This is not the case with the older rocks on which they lie, the strata of which are frequently twisted and broken, and this is particularly noticeable sometimes in the case of the layer of gypsum which is often found between the coral and the sandstone ([Fig. 89] on page 144).
The hills are not marked on any map, indeed no survey has yet been made hereabouts. My account is therefore incomplete, but this does not invalidate the conclusions drawn. From seaward these hills are very easily distinguished from the jagged hills of archean rock, the true boundary of the Rift Valley, by their flat tops and the light yellow colour of their cliffs, and also by their generally being nearer the sea even than the great mounds of gravel which sometimes form the foot-hills of the mountain range.