This band of elevated coral is never very wide, about a mile at Suakin, exclusive of the reef, and rather less at Port Sudan. At Suakin, and to the south it is very slightly raised above sea level, but at Port Sudan and generally to the north it is from 10—20 feet higher, and is separated from the gravel plain by a depression a few hundred yards wide. This depression is often very near sea level and floored with mud in which grow the plants of salt marshes.
Although to the ordinary non-scientific person the idea that most land was once beneath the sea, and nearly all rocks were formed beneath the water, may be known, yet unfamiliar, no one can land on these coral shores without being specially and personally impressed by the fact that, as the ground is entirely formed of corals and shells it has been raised up from the sea, beneath which it was formed. One may walk about on a limestone hill in England and, by patient collection of fossils, partially corroborate the geologist’s assertion that the whole thing is a mass of ancient shells, squeezed together and finally thrust up from the sea bottom, but here, so fresh are the shells, so familiar their forms and so abundant the coral, often complete in all its delicate detail[70] (like those figured opposite pages [88] and [91]) that every man may be his own geologist and assert the origin of the rock as a matter of personal knowledge. Further, he may assert that all the shells and most of the corals[71] are exactly like those now living on the Red Sea reefs, and so deduce the fact that the uplifting of the original coral reef has been geologically recent, long ages since the successive worlds of species of animal whose remains make up the older, and to British minds, more usual, limestones had, one after another, passed away and been finally replaced by the inhabitants of our own world.
It is one of the most recent of rocks, and yet it gives some idea of the meaning of the expression “Geologic time” to remember that these very ordinary-looking shells lived thousands of years before the builders of the pyramids.
We can better appreciate the raw newness of the Red Sea cliffs if we digress a little to the comparison with the very different rocks of Equatorial East Africa and elsewhere which have however much the same origin. These latter are much more typical of elevated coral the world over, the Red Sea, owing to its nearly rainless climate, having peculiarly well preserved the corals of its raised reefs. [Plate XXXV] of cliffs in Zanzibar should be compared with the Red Sea rocks on [Plates XXXII] and [XXXVII], and the comparison made in Chapter VIII, [page 111], referred to. Of course the differences between the elevated coral of the Equatorial coast and that of the Red Sea might be due to the former being of greater age; though, if such a difference exists, as it is not considerable[72], we are led to lay more stress on the different physical conditions under which they are placed. These are that the equatorial rocks are exposed to a considerable rainfall, and, owing to the tides, to far more drenching by spray than are those of the Red Sea, resulting in the solution of the surface layers of the rock and the crystallisation of the dissolved limestone in all the cavities of the interior, thus making the rock both crystalline and homogeneous within, as before described.
Where the Red Sea rock is exposed to alternate wetting and drying the beginnings of this change are evident. All the way along the coast from sea level up to a few feet above that portion of the cliff which is undermined by the waves, the rock is harder and more homogeneous, but this is merely a local alteration due to the action of spray. I had opportunities of examining the internal structure of these cliffs both when the foundations of the quay walls were being dredged out and when the slipway was excavated at Port Sudan. In both cases I found that within the homogeneity of the outer crust disappeared completely, giving place to exactly the structure of a recent growing reef, the larger colonies of coral forming great boulders bedded into a loose mass composed of smaller species and the broken fragments of those more delicately branched. At a depth of five metres I picked out shells retaining almost perfectly the colours and appearance of their living relatives of the same species. The general colour of the excavation was grey, the colour of the mud which is formed by the disintegration of coral and shells by boring worms, molluscs and sponges.
The finding of beds of coral on the tops of the sandstone hills at heights of 500 feet and more above sea level, and the fact that dead coral and shells form the ground along the coastline, are explained, as we have seen, by a general uplifting of the whole country whereby coral reefs have become dry land and even hill tops. The breadth of the maritime plain is another evidence of the same fact, for no such plain can be formed on a sinking coast-line; in such places the successive deposits of sand and gravel from the hills are submerged and the following form layers on the top of the preceding and cannot be carried out beyond them to form a plain.
As an example of such a sinking coast-line compare Norway, where the hills rise directly from the sea and the valleys have sunk below the water forming the characteristic fjords.
Startling though the thought of such changes of the relative levels of sea and land may be, they are of common occurrence, and always have been, in fact they are the commonplace of geological history everywhere. Our present case is a movement of very minor degree, involving but a few hundred feet, a mere detail of the opening of that stupendous fissure, the Rift Valley, of which the whole Red Sea is but a portion.
It is interesting to note how very regular this elevation has been, entirely without twisting or contortion of the strata, so that the individual corals remain exactly in the same position, relative to the surrounding rock, as they did when growing on the reef.
Hence also the almost perfect level of the coast-line, which, spite of “faulting” by which a few small areas rise to a higher level as hills, and the opening of fissures, preserves the same level, within 20 feet, for several hundred miles. At the same time the elevation has been effected in several stages, as evidenced by the existence of level parallel lines of cliff along the sides of hills, e.g. Jebel Zêt in the Gulf of Suez, Jebel Makawar on the Sudan coast, which were cut out by the sea when the hills were at lower levels, and by successive beds of coral at different levels on hill sides in positions that could not be due to tilting of the hill during elevation. Also, at various sheltered points of the shores of Port Sudan and Suakin harbours, and elsewhere, the latest stage of this elevation can be traced in the form of a low cliff, standing a few yards back from the sea, fronted by a reef flat now dry land, though only a foot or two above the sea level. The cliff is undermined exactly as are those still under the influence of the waves, and even the detailed marking of the rock surface characteristic of this marine erosion remains, not yet obliterated by the flaking away of the surface through the action of the sun’s heat and the cold of the clear nights, or by the filing action of the sand blasts of summer.