Desert and water gardens of the Red Sea

by Cyril Crossland

At the seaward edge of the reefs, the coral in the purer water could continue as it is doing now, a rapid and continuous growth, the uniformity of the conditions producing the remarkable unbroken reef already described. The disintegrating forces described in Chapter VIII break the inner side of the reef into sandy flats and pools, but the extension of these pools into wider spaces and continuous channels makes coral growth again possible in the shallows on their banks, so that all the inner sandbanks and rocks are capped with live corals which prevents their further demolition and gives the steep-sided canal-like form to the passages between them.

Living Reefs.

In detail the living reefs of the Red Sea are characterised by (1) their luxuriant growth of coral, (2) the absence of large stones or “negro-heads” along their edges, (3) their frequently crescentic or circular forms. There are remarkably few points on the coast where coral is not growing in abundance, and at the edges of reefs in the open sea its luxuriance is wonderful. Even at the inner ends of the canal-like harbours, where the water is stagnant and dirty, scattered colonies are found. The absence of rivers probably favours these growths, but even so the floods bring fresh water into the harbours occasionally, and colour their waters a deep red brown, like the Nile in flood, for several days together once or twice a year. The absence of strong currents carrying mud is another factor; where such occur, in Khor Dongonab, the vertical sided coral reefs give place to sloping bottoms of rock covered with stones formed by the growth of lithothamnia—stony seaweeds—but I only know this one place where these conditions prevail.

There are no stones of any size on the edges of the reefs, nothing but a few pieces of coral, a foot or so in diameter at most, project above lowest water level. It would decrease the danger of navigation in dead calm weather, which at present, except in frequented places, is considerable, if larger stones, such as the “negro-heads” of some reefs, were present at intervals. As in most cases these larger masses are the remains of former land and are not, as sometimes stated, fragments of the living reef thrown up by storms, one cannot expect their presence here, where the reef edge has grown up in situ. But even large boulders such as form the “hurricane beach” of Pacific atolls, and are cast up by storms in profusion on the reefs of Queensland, are not found here. There is plenty of strong wind in the Red Sea, but the strongest of all, the hot winds of summer, blow off land and do not extend far to sea and such vast breakers, up to 40 feet high, as are recorded of the Pacific, never occur. The reef edge above the precipice consists of a slope of stunted coral which above water-level changes to a gravel of coral fragments coated with lithothamnia; within is sand, generally with bands of seaweeds and marine flowering plants (grass-like in appearance) on the shallows, with muddy pools and channels a fathom or two deep. The edge is higher than the rest, but does not appear continuously above lowest water-level. Reefs not exposed to the waves have not this definite edge, the platform being covered with two or three feet of water in which stand separate coral colonies, numerous at the edge, rarer within towards the lagoon.

The crescentic or circular forms of isolated reefs is very far from being a peculiarity of the Red Sea, but is worth while mentioning as bearing on the formation of the lagoons of atolls on the larger scale. From the smallest to the largest these reefs shew a hollowing out in the centre, where the reef material, not being protected by living matter, is exposed to the destructive influences detailed in Chapter VIII.

There are some reefs in the Red Sea quite of the Atoll form, of which the largest is Sanganeb, the plan of which, on the map on [page 137], is sufficiently explanatory. Another is Tella Tella Seghir, which is elevated to 40 feet above sea-level, and consists of a ring of high ground enclosing a depression, once a lagoon though its floor is now a little above sea-level. The elevation of this ring-shaped reef has been at least two stages, the lagoon having contained water to about one-third of its depth comparatively recently, since a line of undermined cliffs occurs at this level. The Admiralty Pilot remarks that the edge of the ridge bears numerous cairns. Some of these are artificial I am told, but those I examined were large coral colonies, in the position in which they grew, left exposed by the removal of the softer stuff in which they were embedded. One example of the coral genus Mussa was especially conspicuous, as much so as the corals illustrated on [Plate XXXVII] on the top of Jebel Têtâwib.

Summary of Geological History of the Red Sea.

(1) There was originally a shallow sea covering the space between the high mountains of both sides of the present Red Sea. In this sea were laid down sandy and gravelly sediments, and limestones were formed which are now found in the sandstone hills of the maritime plain, &c.

The gypsum beds found here are the result of the drying up of the water of this shallow sea.

(2) The beds of rock thus formed were broken up by the sinking in of a long strip of the earth’s crust forming the Rift Valley, which extends from Jordan to Tanganyika. Part of this valley was filled by sea water and became the Red Sea.