Desert and water gardens of the Red Sea

by Cyril Crossland

Diagram 3. Further Growth of the Reef

the support of the still growing reef above? Diagram 3 explains. Passing seawards from F is the gentle slope formed by the breaking waves, next a precipice, followed by a very steep slope to the sea bottom beyond A formed of the broken and dead corals fallen from the growing zone above, and which forms the foundation on which the shallow water corals extend the reef seawards.

Diagram 4. The Abrasion of the Coast

Now at the same time as growth has added to the reef seawards the waves have cut down the land on the other side. Consider this case separately and then combine with that above. As before, A, B in Diagram 4 is the outline in section of recently formed land upon which the sea has as yet had no action, and C, D is the level of lowest tides, C′, D′ that of the highest. Between these two levels, upon the land mass lying between D and D′, is the never ceasing beat of waves and the wear of silt-carrying currents, so that in time the land is eaten away along a line a little above C, D, say X, Y, and a cliff, Y, Z, is formed. We are assuming that the material of the land is sufficiently coherent to form such a flat and cliff, but even so in general X, Y becomes a sloping shore, not a flat. It only remains as a flat if for some reason the seaward surface at X is protected against further detrition by waves, e.g. by the growth of corals and stony seaweeds. If they are present, even if their growth adds nothing to the mass of the rock, it hinders its decay and causes the formation of a reef flat in place of a sloping shore.

Diagram 5. Formation of fringing reef partly by growth of coral, partly by cutting down of land

Now these two processes, addition by growth and abrasion by wave action, go on simultaneously, and to get at the true method of formation of a reef flat in the Red Sea the two diagrams must be combined, as in Diagram 5, where as before F is the raised reef edge and F, Y the whole extent of the reef flat, and the cliff Z, Y is undermined as shewn. Where, as in some seas, F to D is recent growth and X to Y is rather older coral rock, it is impossible to locate accurately the dividing line between reef formed by recent growth and that cut out of the land, but near shore, where the flat is free from mud and sand, its surface is seen to consist of sections of the constituent shells and corals, cut as cleanly as if done by a stonemason (see [Plate XXXIII]). Even such hard shells as those of the “giant clam,” Tridacna, are cut across at the same level, thus shewing very clearly the origin of the surface by the planing down of a mass of rock to that level.

The boat channel indicated between F and G remains to be accounted for. As the reef flat widens it results in a great area covered by shallow water at high tide level and partly bare at low. Exposed to a tropic sun life is impossible for any but a few specialised forms, the rock is unprotected from such wave motion as there is, and from boring organisms, which agencies quickly reduce its level. Strong currents also flow over the surface, for the breakers throw water over the raised edge which may have to travel several miles before reaching a gap through which it can return to the sea, and this with tidal currents cause swift rivers of muddy water to flow over the reef flat parallel with the shore. The obvious result is the hollowing out of a boat channel[50], and the accumulation in it of great quantities of mud and sand, which in many places form the greater part of its actual surface, but which eventually are swept out to sea.

The presence of certain marine flowering plants of grass-like form (Cymodocea and other genera) assists, if not wholly responsible for, the formation of such accumulations by binding the mass together by their strong and tangled roots and rhizomes.