Diagram 11. Fringing and barrier reefs in neighbourhood of Ras Salak, shewing correspondence of both systems with coral formations on land
Soundings with line and dot over mean that no bottom was found after so much line was run out.
The rocks of which these ranges are composed are laid bare in the cliffs which have resulted from the upthrusting of the hills of the Rawaya-Makawar range, and on the hills of the maritime plain.
Plate XL
Fig. 88. In a fault ravine of Abu Shagara. Cliff coral, gypsum and sandstone, the latter containing sheets of recrystallised gypsum, selenite, in every crack
I illustrate overleaf part of Jebel Têtâwib in the north part of Rawaya. It is about 40 feet high, and of this from one to six feet are occupied by the basal sandstone, a soft laminated rock generally yellow in colour, sometimes greenish or red. Next is a band, up to 20 feet thick, of gypsum, the strata of which are considerably contorted in contrast to the coral formations overlying them, which are nearly horizontal, and as usual retain the relative positions they occupied during the growth of the reef. In the south Jebel Abu Shagara is higher, 127 feet, and its cliffs, being higher, contain very much more sandstone, but are essentially the same, as are those of Jebel Makawar and Mayitib, and those of the sandstone hills of the mainland.
The sandstone ranges, coral coast-line, and barrier reefs are then three parallel repetitions of the same structure extending with great regularity along the sides of the Rift Valley from the entrance to the Gulf of Suez to Suakin, a distance of about 700 miles. Southwards of this point, as we shall see, similar structures occur, but without this extreme regularity.
Their formation is due to the opening of the Rift Valley which resulted in these sandstones[73] being thrown into a series of steps as it were along each side of the trough, as shewn on [page 145]. Of these we are acquainted with three, but more would probably be discovered if detailed soundings were taken from outside the barrier reefs to the narrow trough which runs down the centre of the sea and is a thousand fathoms deep.