The further history of these three steps or ridges has been as follows. We will distinguish them as numbers one, two and three, the former being the highest, the present sandstone hills and ridges of the maritime plain. The coral caps on these were formed when the sea reached to the bases of the Archean hills, the sandstone range No. 1 being a line of barrier reefs off the mountainous coast-line. The mountains, then as now, were being broken down by the action of the weather, and the resulting sand and gravel was washed down into the sea as the beginning of the maritime plain.
Fig. 89. Jebel Têtâwib, in Khor Dongonab, a butt near its southern extremity, seen from its south side.
The letter B is at a level about 25 feet above the foreground
- A. Coral colonies, in position of growth, bedded in a mass of loose coral débris, shells, &c.
- C. Hardened coral-mud. The weathered surface forms rounded masses in low relief.
- D. Gypsum strata, here steeply tilted, and upturned at their ends in the piece shown in the foreground. They are much folded in other parts of the cliff.
- E. Green and red shaly rock underlying and sometimes interstratified with the gypsum. It is here broken down into sand. This rock contains sheets of glass-like recrystallized gypsum.
Meanwhile organic remains were accumulating on ridge No. 2, and as elevation brought this within fifty fathoms or so of the surface, reef corals took possession and covered the summits one after another as elevation proceeded, so that
Diagram 12
when ridge No. 1 emerged from the sea altogether, and its bases were surrounded by the gravel from the hills, No. 2 was a second barrier reef out at sea.
The same process has been repeated, so that coral growth and levelling have made ridge No. 3 into the present barrier system and the maritime plain has reached the one-time barrier No. 2 and so made this the present coast-line.
During the last of these elevations a good deal of breaking and cracking of ridges Nos. 2 and 3 took place. For instance, Rawaya was originally connected with the mainland, the proof being the presence upon it of scattered pieces of Archean rock which could not possibly have reached it unless a continuous surface stretched from it to the old hills. Dongonab Bay, and with it probably other parts of the channel within the barrier system, have evidently been formed or enlarged since the maximum extension of the maritime plain. The harbours of the coast, which are so interesting in themselves as to deserve separate consideration, were formed also at this time.