Chapter IX: Geography of Red Sea and Foundations of the Reef Systems (1)

_The Climate_ has already been roughly described but it is interesting enough to deal with in more detail.

One might suppose the extremes of dry heat and cold of the desert climate to be moderated by the sea, and the resulting mean to be a fairly mild and equable climate. Actually we get alternations of desert and sea climates, extreme dry heat in summer and steaming winds from the sea, both bringing great discomfort.

The winter from November to March is cool and pleasant so long as the prevailing north-east wind blows, but there are spells of very disagreeable weather even in winter. When the wind is from the south-east the temperature rises and at the same time it becomes very damp, saltish moisture being over everything, so that even the natives become lazy and depressed and many suffer from rheumatism, &c.

One has however the satisfaction of knowing that the south-east wind usually lasts but three days or so, and never more than a week, when the north wind comes back and we revive.

The south wind is generally preceded by a day’s calm and increases in strength until the end, when a short calm ushers in a very strong wind from the north. On several occasions I have actually seen the approach of this sudden and welcome change as a line of low cloud, formed by the condensation of vapour where the cold north wind meets the damp from the south.

This sudden change was the cause of the wrecking of a _sambûk_ which was beating down to Port Sudan in a south-east wind. Anchoring one night in a long narrow harbour open to the north, they were caught by the north wind next morning and, being unable to beat out against it, were driven on to the reef. The crew had to walk in to Port Sudan, distant about twenty-five miles, without food or water, one of them having a badly crushed wrist. As I had cargo on the _sambûk_ I went up immediately, and after only two or three days there was nothing visible of the _sambûk_, but fragments scattered over miles of reef.

In winter the desert wind, due north or a little west of north, is very much colder than the usual north-north-east The mornings indeed may be quite chilly, and though this is very welcome to the Englishman the natives suffer considerably. On the first day or two of such a period the wind is strong, charged perhaps with sand, and so dry that the backs of books curl as if they had been before a fire.

In the summer the alternations of climate may be astonishingly rapid, both may occur on one hot-weather day in July or August as follows. The land breeze is very weak, and dies away about 6 a.m., when already the sun is blazing hot. By 8 a.m. it is intolerable, but as it is still dead calm pearlers and fishermen are at sea making use of their opportunity. If however they expect a day of “hurûr” or hot wind they do not go far away, and when warned by two or three preliminary puffs of wind off shore, they must make all haste to return, or risk being swept out to sea. In half an hour the wind may be furiously strong, heated as by a furnace and bearing dense clouds of fine dust, of the colour and density of a London fog, together with coarser sand that stings the face. Woe to one who has to travel against such a storm! The dry heat soon produces intolerable thirst, the eyes, nose and mouth are filled with sand, while one’s face, eyelashes and even teeth are caked with mud produced by it with the natural moisture.

These conditions continue until noon, when a change may be expected, but may be deferred until 4 p.m., or rarely even 6 p.m. The wind suddenly ceases, the world becomes again visible, and the temperature drops from say 105° F. to 95° F. But soon there comes the reverse wind, almost equally strong, from the sea, and the humidity increases so much that the fall of temperature is not the relief that might be expected, being but the change from oven to steam-kettle. The natives tell me that this wind, so hot in the plains, among the mountains is cold, and is heated by its passage over the sun-roasted plains. Apparently the great heat here originates miniature local cyclones, cold air from the mountain tops, or drawn over the mountains, rushing down to fill the low pressure area on the plains, being heated there and rushing on a few miles out to sea, whence the easterly return wind originates. At Dongonab these “hurûr” winds are rarer than they are further south, where they are of almost daily occurrence during the summer, while at Halaib, 100 miles further north, the natives tell me they do not occur at all. Consequently we are sometimes visited by the return wind in the morning, caused by “hurûr” at a point further down the coast. Such a cyclone is illustrated by the frontispiece, which represents the combination of thunder clouds over the mountains while a “hurûr” rages over the plain and for several miles out to sea. But among the barrier reefs, though the wind is blowing directly towards them, all is glassy calm.

The rainfall is extremely scanty and local, though markedly better in the south, where the population is correspondingly greater and the fauna richer.

There are two seasons when rain may be hoped for, viz. the “kharîf” which centres round August, and which is referred to in the frontispiece, and the winter months, but if rain fell for an hour or two on three days it would be considered a liberal supply for the whole year in most places. At Dongonab there has been no rain (above a millimetre or two) since December, 1907, though one or two showers have fallen on Rawaya and Makawar[59]. There is of course much more rain on the hills than on the plains, but even so grass grows only in scattered areas to which the people migrate.

_Tides._ The Red Sea undergoes considerable variations of level at its extremities, up to seven feet at Suez, but in the middle the variations are small, only a few centimetres at Port Sudan. At Dongonab the difference between highest and lowest levels recorded is 80 cm., but the maximum change in any 24 hours is rarely over 30 cm. Records shew a distinct tide, but this may be interfered with by changes of level due to wind and changes of atmospheric pressure, and in any case one of the usual two tides of the 24 hours is practically suppressed, the water remaining near high tide level until it falls for next day’s tide. In the summer the average level is lower than in winter and the tidal effects are partially masked by the results of the peculiar climatic conditions. The water may remain low for days, so that all the coral which has grown above that level since the last occasion of extreme low water, which may have been one or even two years ago, dies off.

I suppose that every school-boy looking at an atlas, is struck by the peculiar shape of the Red Sea, and is led to ponder on the usefulness of this peculiar canal, the sole value of which is that it gives communication between Europe and the East, a value which needed but the trifling addition possible to human effort to make it the great highway of the world. Its own shores are desolate wastes, in itself it has no attraction for traffic, and even its shape seems to indicate that it is but a passage to other seas. (See map inside the cover.) For so narrow a sea, only a little over a hundred miles wide, the depth is great, two hundred to five hundred fathoms at the side and a thousand in the middle. These peculiarities are also well marked in the deep Gulf of Akaba which bounds Sinai on the east—the Gulf of Suez, on the west, being a shallower branch valley. Both these gulfs, like the Red Sea, are bounded on either side by high mountains, and those of the southern part of Sinai are particularly grand in the savage barrenness of their jagged peaks and vast precipices.

The Gulf of Akaba is directly in line with the Jordan Valley, a similar depression on a smaller scale, only partially occupied by water, the Dead Sea, while southwards we find another dry valley running through British East Africa and adjoining territories, a great trough bounded by plateaux, several thousand feet above its bottom. We can thus trace

this trough-like valley from Palestine to some degrees south of the Equator as a stupendous crack in the earth’s surface, well named “The Great Rift Valley[60].” The Red Sea is its greatest section, its total depth here being, say, 5000 feet from the summit of the mountains[61] to sea level and 6000 feet to the sea bottom, 11,000 feet in all.

The formation of such a valley, by the dropping down of a series of strips of country below the level of the remainder, is illustrated by Diagram 7. To study the simplest possible case we draw a section through the ground and imagine it formed of three kinds of rock, of which two form horizontal sheets, AA and BB, over the third CC. These were originally unbroken, and in the positions shewn by the lines of dashes, but were broken by the dropping down of the central part to form the valley shewn here in section. The floor of the valley has the same structure as the original surface of the ground, the same three beds, A, B and C, occurring in the same positions, but at a lower level. They are found again in each of the steps on the valley’s sides, their regular reappearance in this way being conclusive proof of the earth movements postulated.

The vertical lines FFFF, between each step and the next drop, along which the continuity of the beds is broken, are termed “faults,” a geological term which should be remembered.

Rift valleys are found elsewhere in the world, but are exceptional, ordinary valleys, with their winding courses and rounded outlines, having been formed by the action of streams, which slowly wash away the ground and hollow out their courses to the sea.

The actual structure of the middle portion of the Red Sea Valley is shewn diagrammatically by the section on page 145. Five steps are shewn, Nos. 2 and 3 being further separated by a minor fault valley. The details are described later.

The southern part of the sea, below Masawa, has recently been subjected to volcanic[62] action; many of the islands there are quite well-preserved volcanic cones, but as regards the rest of the sea, though earth movements have been frequent and considerable, there are now no traces of volcanic action, and the movements that have occurred have not necessarily involved cataclysms greater than severe earthquakes.

There are however in the north two islands the existence of which is most readily explained by volcanic action. I refer to the coral formations known as “The Brothers” and “Daedalus Shoal,” the former a pair of low islets, the latter a flat reef, rising out of the centre of the sea and surrounded by water hundreds of fathoms deep. They are extremely steep-sided cones, and what could form and support such structures far out from land is puzzling. A certain view of another section of the Rift Valley, that once seen can never be forgotten, seems to offer an explanation. After passing through the forests of the Kikuyu Plateau by the Uganda Railway one comes out into the open on the brink of the great escarpment of the Rift Valley and looks across a trough 3000 feet deep to the similar forest-clad heights of the Mau on the other side. The continuity of the valley is rudely broken by two volcanic cones rising abruptly in the middle of the flat bottom of the trough. On consideration the strangeness of their appearance in the middle of a valley passes away, one sees that the bottom of such a rift must be a zone of weakness of the earth’s crust where volcanoes might naturally be expected to arise.

If the water were removed from the Red Sea Valley would not the appearance of The Brothers and Daedalus be very much like that of the two volcanoes of British East Africa, allowing for the steeper angle at which their materials would lie under water? Given such cones of loose volcanic ash, &c., wave action would quickly level down their summits until coral growth afforded protection and formed a cap of rock, part of which is now raised again above sea level as the islands on one of which the lighthouse is built.

The ring-shaped reef of Sanganeb[63], opposite Port Sudan, which is outside the Barrier system and separated from it by water 400 fathoms deep, may be built on a similar foundation. Like the two coral reefs above it rises with extremely steep slopes from this deep water, and is the summit of a submarine pinnacle rather than hill.

On the other hand, the foundations of these strangely isolated reefs may be like a certain island which, rising high above sea level, shews its structure, a centre of olivine rock fringed with coral. This island is variously known as Zeberjed, St Johns, and Emerald Island, the latter name due to its possession of mines for peridots, which are worked by the Khedive of Egypt. Its position is 23° 30′ N., distant about 60 miles from the African coast, a formation quite independent of the sides of the Rift Valley. It is an example of the “Block Mountains” described by Professor Gregory, portions of the original earth surface which have remained standing when the surrounding country dropped down to form the trough of the Rift Valley, not a mass of land thrust upwards and subsequently carved into peaks and valleys by running water, which is the way ordinary mountain ranges are formed.

One gets a good idea of the structure of the Red Sea coasts on leaving the Gulf of Suez for the voyage south, before the ship’s course passes far from land. On the western horizon is a range of wild mountains, a grey plain ending in a yellow shore-line separating them from the sea, and the off-lying islands are of the same colour. The plain is formed of gravel from the high hills, its yellow border seawards being coral limestone, and the islands also. In the sea are numerous reefs, here of very intricate plan, lines of white breakers separating the deep blue black water from large areas of green and brown shoals in waveless lagoons. There are deep channels between these reefs and the shore, which is itself fringed by a shallow reef with its edge at low water level but bearing perhaps one or two fathoms of water on its surface within.

This being the simple structure of both sides of the whole Red Sea trough I may proceed to describe in detail one section of the coast, that bounding the territory of the Anglo-Egyptian Sudan, between 18° and 22° N. This section includes two (Ras Rawaya and Ras Salak) of the three promontories which break the straight line of the west coast north of Masawa, the third being Ras Benas, further north. The map opposite shews clearly the fringing reef which lies along the whole coastline, the numerous harbours, of which Port Sudan, Suakin, and Trinkitat are of commercial importance[64], the deep channel separating the fringing from the barrier reefs, and the atoll of Sanganeb on which the lighthouse is built.

Sandstone hills shaded, _small_ islands black. Coastline double, the outer line being the edge of the fringing reef. The thin lines enclosing roughly oval or elongated areas at sea are the barrier reefs. Figures on sea represent depths in fathoms.]

On land the bases of the high mountains are indicated, and certain lower hills, of sandstone, which rise in the midst of the maritime plain. A striking fact is visible on first inspection of this map, viz. that not only is the Red Sea a nearly parallel-sided trough but that the constituents of the sides are themselves placed in lines parallel to the coast. The Archean hills[65], the lesser sandstone ranges, the coral bounding the maritime plain, and the barrier reefs, are all four roughly parallel to the main axis of the sea.

We will consider each feature in more detail. For the Archean hills consult the extremely interesting memoirs of the Egyptian Geological Survey[66]; for our purposes it is enough to note that they are all of ancient igneous and metamorphic rocks, that they rise to heights of from four to eight thousand feet, and the valley bottoms are generally flat and filled in with gravel.

The maritime plain is from five to ten miles wide, sloping up regularly from the sea towards the bases of the hills, where it may attain a level of several hundred feet. Except at its seaward edge, it is composed of black gravel, the product of the decay of the hills carried down by the torrents resulting from the rare but furious rain-storms, and spread out to form the plain. Sand-hills occur, but not very commonly, though the gravel is mingled with sand throughout, and in sections of the plain exposed by wells, layers of gravel alternate with sand, fine or coarse, as far as the deepest borings have been carried[67].

The pebbles, though black predominate, are of a most remarkable variety of kinds and colours. Bright green and red, yellow and clear white are abundant, and any square yard would yield a rich collection in Petrology. As the torrents open out into the level plain they lose themselves, continually taking to fresh channels, so that the _débris_ from series of hills quite distant from one another are mingled; in a given spot gravel from one valley is laid down this year, from another and totally distinct one another. One would expect gravel which had been carried by torrents a distance of many miles to be rounded down by friction into smooth boulders or pebbles, like those of our home streams. As a matter of fact it is nearly always angular, the rounded surfaces we should expect being rarely met with on the surface. The pebbles, as we now see them, have been re-formed from larger stones since their transport through the valleys and over the plain. Large stones, lying half buried in smaller material, shew the usual rounded surfaces of water-borne rock, but they are invariably split up by fissures, which may be half an inch broad, so that the stone is as it were built up of angular fragments fitted together after the style of a puzzle picture. During the hundreds of years they have lain there, apparently secure from all interference, they have been exposed to innumerable fierce heats and cold nights, which, causing successive minute expansions and contractions, have at last split the stones into small pieces. This is the origin of the irregularly shaped gravel; first indeed it was rounded by the grinding and pounding of the torrents of hundreds of successive winters, then it was split up again by the silent invisible stresses of heat and cold.

_Plate XXXVI_

Yemêna oasis in foreground and gravel-covered ridge across middle distance]

_Plate XXXVII_

The accumulation of this vast mass of gravel and sand in the manner described has taken a length of time compared to which a human life is but a moment. Even from the geological point of view it has been not inconsiderable. There is abundant evidence that the plain was formed, much as it is now, at a time when the coast-line was entirely different, and though there is no good evidence of the country’s having been other than desert throughout historical time, there probably was a greater rainfall when the formation of the plain was in full swing.

The sandstone hills are particularly interesting in that one finds a regular layer of coral on their summits, which shews that they were once nearly level with the sea, and are in fact coral reefs which have been elevated to heights of from 100 to 1000 feet. In some of those hills to which I have had access the corals on the summit are wonderfully well preserved, and by this fact, and that the species are indistinguishable from those now living in the sea, prove the elevation of the hills to have been geologically recent. Further, the larger coral colonies are at once seen to be still in the position in which they grew, not tilted or overthrown in any way. This is not the case with the older rocks on which they lie, the strata of which are frequently twisted and broken, and this is particularly noticeable sometimes in the case of the layer of gypsum which is often found between the coral and the sandstone (Fig. 89 on page 144).

The hills are not marked on any map, indeed no survey has yet been made hereabouts. My account is therefore incomplete, but this does not invalidate the conclusions drawn. From seaward these hills are very easily distinguished from the jagged hills of archean rock, the true boundary of the Rift Valley, by their flat tops and the light yellow colour of their cliffs, and also by their generally being nearer the sea even than the great mounds of gravel which sometimes form the foot-hills of the mountain range.

Passing from south to north the first range is met with a few miles north of Mersa[68] Durûr, as a chain of low butts rising from the alluvial plain a few miles inland. These become higher and more continuous as one passes northwards, culminating in two considerable hills, of about the same height and area of base, the northern of which is marked on the charts, where it is called Table Mountain, and given a height of 1000 feet.

At about five miles inland from Dongonab are a couple of small hills standing alone, but a little farther north lying inland from the middle of the North Basin of Dongonab Bay is a considerable range, extending towards the hills about Khor Shinab, Hamama, &c., from which it is separated by an interval of only a few miles.

The Abu Hamâma[69] range (which I so name from its most prominent though not highest peak, a landmark for sailors) extends from about the inner branches of Khor Shinab to some distance beyond Khor Abu Hamâma, lying much nearer the sea than do the others. Its height is estimated by a government surveyor at from 500 to 700 feet. (Map, p. 126.)

These ranges are wholly inland, and rise from the maritime plain, which they divide longitudinally. The scattered ranges of sandstone are not the whole of this formation however. I give a view of a part of the maritime plain in which it is seen to rise as a distinct fold across the middle distance. This appears to consist of the usual gravel, but where cut into by the Yemêna ravine a very different state of things is displayed. It is practically all sandstone, covered by a few feet of gravel and gypsum conglomerate. Of coral I only saw one large boulder, having no time for a search. There is yet another range rising from the sea, namely two small hills on the peninsula of Rawaya, and the islands of Makawar and Mayitib, of which Makawar is the only considerable elevation. This range is of special interest and will be described in detail.

_Plate XXXVIII_

Two views in Yemêna ravine, which cuts the maritime plain]

_Coral of the coast-line._

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.

As explained in Chapter VIII, page 107, much of the fringing reef is really a part of the coral limestone of the coast, and its formation needs no further explanation, but it is interesting to note how greatly it varies in width in correspondence with the height of the land behind it. For instance, about Suakin it is up to 1½ miles wide, at Port Sudan only one-third of this, in correspondence with the fact that the shore about Suakin is raised scarcely two feet above sea level, whereas at Port Sudan it rises six to ten feet.

This variation is exactly what we should expect on the theory of reef formation by abrasion, the cutting down of the low-lying land involving the removal of comparatively small masses of rock and so proceeding quickly. Again, on the coast about Ankêfail (see Map, p. 139), where the land is as high as at Port Sudan, the reef is only a few yards wide, but this may be partially attributed to the shelter from waves provided by the large island Makawar.

That the differences in breadth are so marked shews that abrasion has had much more to do with the formation of the reef flats than has growth of coral, for we see no reason why this latter factor should not have operated equally well over the whole coast and tended to equalise the reefs’ breadths.

But it was also explained in Chapter VII that ordinary corals cannot grow in very deep water, and as we find depths of even 200 fathoms just over the edges of these reefs we are confronted with a problem. We have explained the origin of only the surface of the ground, what lies beneath and how it came about that there was a foundation ready, within the narrow limits of depth in which corals could build, continuous through so many hundreds of miles, are the real problems. The barrier reefs, right away from land, from which they are separated by deep water, still more conspicuously need an explanation. These are the main questions of this chapter.

_Barrier Reefs._

The barrier system is not a single linear reef, or line of reefs, but rather a line of areas of shallow water full of reefs of all sizes, generally more or less crescentic or ring-shaped. The details have not been surveyed, except very partially in some cases, the charts from which the map on page 126 is copied merely giving the outlines of the areas on which the reefs stand.

Some of these areas are very broad, the southernmost, Towartit, being eight miles across, their size, intricacy, and their being completely useless to all navigators but a few pearl fishers, preventing their survey within the outer borders, except in the case of that which bounds the passage to Port Sudan on the north, shewn on the map opposite. This area is obviously a continuation of the barrier system, spite of the fewness of its reefs, and the fact that over the greater part of it an average depth of 10 fathoms obtains. It is a young reef, mostly not yet grown to the surface.

_Origin of Barrier Reefs._

The origin of these reefs cannot be explained by any of the theories discussed in Chapter VIII. Darwin’s theory is quite inapplicable as the coast has risen continuously throughout recent geological time, and no currents could have carved out such a channel as that separating the reefs from the land, with such irregular great depths as are shewn on the maps, where, within the barrier depths over 150 fathoms are seen in proximity to soundings of only 30 to 40 fathoms, or even close alongside surface reefs.

Finally, coral growth alone, as already mentioned, could not give rise to such sheer precipices as those in which these reefs generally end.

The examination of two features of the land makes all clear at once. These are the promontories and sandstone hills, of which the Rawaya peninsula is the best example, and worth describing in some detail.

The map opposite shews that Rawaya is a large area joined by a very narrow neck to the mainland and enclosing a large bay, Khor Dongonab, about 20 fathoms deep.

Directly south of its extremity are the islands of Makawar and Mayitib, which, like Rawaya, enclose a deep basin of water (40 fathoms) on their west side, while on the east depths of 200 fathoms are found only a mile and a half from Mayitib, and 300 fathoms only three miles away, while half a mile from the islet of Shambaya the same depth occurs. In comparison the elevation of the peninsula, and even of the islands, is very trifling, and the difference of level between them and the maze of reefs which separates them absolutely negligible. Indeed

Rawaya is extremely low, its average being about ten feet above the sea, the areas of its two hills, Jebel Têtawib in the north, and Jebel Abu Shagara in the south, being inconsiderable, and their heights only about 40 and 127 feet. Further, an inspection of the ground shews that these hills are merely parts of the peninsula which have been thrust up to a higher level (see Diagram 9), and even on Makawar, where much of the island attains a height of over 250 feet with summits of 300 feet, the two ends and west side are low like Rawaya. In short, Rawaya, Makawar and the reefs between and about them are obviously one continuous ridge, the middle part of which is slightly lower, and, by coral growth and wave erosion, has been built up and cut down into the level area of reefs we now find there.

The narrow fringing reef along shore of mainland shaded as are coral beds in Khor Dongonab. Small Islands are black. Dotted areas are reefs free from coral.

-·-·-·-·- 10 (and in south basin 20) fathom line.

--------- 100 fathom line.]

The conversion of such low-lying land as Rawaya into a reef maze follows at once from the action of the sea, restrained by coral growth, described in Chapter VIII, but the diagrams make the case clearer.

A is the first stage, the thin line representing the outline of a partly submarine hill range, the undulations of which are much exaggerated in the diagram. The horizontal dotted line is the sea-level, so the diagram represents one summit above the sea, an island, another submerged, and a third emerging to the right.

A First stage. B after elevation and second period of abrasion.

----- sea level.

[Legend: wavy lines] additions made by coral growth.

Coarse shading = remains of original hill after abrasion.

Fine shading = coral mud or sand.]

In A the first summit to the left appears above water as a rounded island, which is cut down considerably and much of its area converted into reef flat, and the deep lagoon to the right is narrowed by coral growth on both this reef flat and over the next summit, as indicated by zigzag shading in the diagram.

As this second summit is at about the right depth below the surface, coral grows vigorously upon it forming a surface reef, slightly hollowed out in the middle. The third summit is like the first.

_Plate XXXIX_

Upper figure in the north of the peninsula, lower near salt works in the south. Both shew one of the canal-like inlets of the sea which cut up the western side of the peninsula. In the lower one the inlet is partially cut off from the sea and the great heat has evaporated its water leaving a lake of salt. The low ground of Rawaya is shewn behind as a mere line on the horizon in the upper, but in the lower photograph the hill of Abu Shagara is included.]

In B the thin line represents the final stage of A, further elevation and abrasion, with coral growth, resulting in the levelling down of islands and reefs and partial filling of the deeper lagoons as shewn by the shaded area of the diagram.

Summit No. 1 is not only cut away altogether but hollowed out into a shallow lagoon, the deep lagoon has been narrowed considerably while the ring-shaped reef on summit 2 is much as before, but has spread out and encloses a larger lagoon, thus becoming a small atoll. Summit 3 shares the fate of number one.

Now compare the outline of the original hill range in A, with the shaded line in B, and the levelling action of the sea, both upwards and downwards, is evident.

This explanation of the origin of the reefs between Makawar and Rawaya can obviously be extended to those to the south as far as the Têlat Islands, and to the whole barrier system in fact. The reefs south of Salak are similarly related to a large area of raised coral extending from the point northwards, and though there is no bay here, corresponding to Khor Dongonab, there is a large salt marsh separating this from other raised coral to the west, and formed by the filling in of a bay by blown sand. The diagrammatic map overleaf makes this clearer, and shews that on land we have continuations of both kinds of reef, the barrier being continued as the eastern coral ridge, the fringing reefs of Salak Seghir being one with the limestone on the west side of the swamp. Similarly Ras Benas to the north (lat. 24° N.) and the angle at the entrance to the Gulf of Suez, have reefs and islands in continuation of them southwards, the former being named Makawar, in this as in appearance recalling the island off Ras Rawaya.

It is now evident that the origin of both barrier and fringing reefs is identical with that of the whole coast-land, and is not to be looked for in any laws of coral growth, or marine sedimentation and abrasion, these factors having merely affected the summits of submarine hills hundreds of miles long, nearly two thousand feet high, often peculiarly narrow, and always more or less parallel to the axis of the sea-filled Rift Valley.

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_

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.

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.

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

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.

Natural harbours, almost completely surrounded by land or reef, waveless in all weathers, more perfect than almost any made by man, abound throughout the length of this coast. In one part, just north of Rawaya, are ten of these strange inlets in a space of only 40 miles (see map, page 126). That of Suakin has been already described; on entering for the first time it is hard to believe that this long parallel-sided, deep channel, bounded by reefs covered only by a foot or two of water, and then by land only the same amount above the sea, is not an artificial canal. It leads nearly straight inland for two miles, but not _quite_ straight, indeed there is a bend that large steamers frequently fail to clear, and which led to the abandonment of Suakin as the Port of the Sudan.

Obviously this canal-like inlet is not the mouth of a river, past or present, for present rivers there are none, and no river, flowing over a wide plain, through loose and heterogeneous materials, could cut out such a channel, but would end in a wide shallow estuary or delta, if it formed a definite mouth at all.

The new harbour of Port Sudan is much wider both in the entrance and within, but the origin of this deep landlocked basin is equally puzzling.

The forms of all the harbours of the coast can be reduced to one plan more or less easily, that of a cross with arms parallel and at right angles to the coast-line, and are in fact formed by two cracks in the earth’s surface nearly at right angles. The former arm is generally the largest, in Port Sudan it is two miles long, the other arm, which connects this with the sea and forms the shallower branch harbour, being much the shorter. The same applies to for instance Wiai, Fîjab, Salak Seghir and Ankêfail Kebir, whereas in the case of the narrower harbours, like Suakin, Arûs, Shinab and its neighbours, the arm at right angles to the sea is the longest, and the plan of the inlet is more like the conventional cross.

The arrow in Khor Shinab indicates the point at which the material of the cliff changes from coral to gravel.]

In Wiai, Fîjab, Salak Seghir and other harbours most of the land between the inner arm and the sea, corresponding to the East Town in Port Sudan, has been cut down

and converted into reef, upon which strips of sand have accumulated to form islands in places. In all three harbours currents flowing in and out have buried the south end of this reef, next the entrance passage, in a steep sandbank (the point is marked by an arrow on the plans). As the water is too deep for convenient anchorage of small vessels, the _sambûks_ run their noses on to these sandbanks, a couple of sailors walk ashore with the anchor, and they are moored for the night, as the prevailing wind is from the north.

Salak Seghir has a long narrow winding entrance, like a deep still river between reefs. Having successfully but fearfully navigated this in my launch, I found that my sailors’ design was to run her on to the sandbank, _sambûk_ fashion. I declined this, for my copper sheathing’s sake, and was all unprepared for the fact that the passage there is about as wide as my launch is long and that the inner branch is shallow and full of humps of coral, giving me a choice of evils which I do not intend to make again. The sand lying on the reef between the inner harbour and the sea has become consolidated into sandstone in a narrow parallel-sided band, perfectly level and almost as regular as an artificial breakwater. A short length of such a formation would be striking, but this extends to nearly two miles.

These curious and most useful splits of the land have been made since the maritime plain was complete, as we saw was the case with Dongonab Bay, and, consequently, part at least of the barrier system. In some cases the innermost parts of the harbours are composed of gravel, not elevated coral. At Fîjab this is due to erosion of the coral, as shewn by rocks and islets of this material remaining on the shallows which separate the gravel cliffs from the deeper water, but in other cases the gravel bounds the actual fault.

This is well seen at Shinab, where almost the whole harbour is bounded by raised coral cliffs, but near the innermost end this is overlaid with gravel, and finally gravel replaces the coral in the most regular manner, shewing that the two materials were in perfect continuity when the split which made the harbour occurred. The north and south limbs of the crosses have been largely filled in with water and wind-carried sand; they were originally of much greater length.

The peculiarities of the coast impress unusual methods upon those who travel along it by sea. The wave motion varies greatly; from Port Sudan to Darûr for instance the waves are much the same as on an open sea, from Darûr to Fîjab the barrier system gives considerable shelter, the vessel passing into perfect calm for short periods as she approaches near the reefs in tacking. From Shalak to the Têlat Islands is a bad bit in stormy weather, quite open sea and no possibility of anchoring anywhere in an emergency, so that vessels are often windbound at Salak[74] anchorage, waiting a fall in the wind in which to reach the next section of the barrier system.

To travel by night is obviously impossible, the navigation of a boat, even among well-known reefs, when moonlight seems bright as day, is an experience once tried never repeated, without urgent cause. Even when the sun is low it is extremely difficult to see one’s way, though a good native pilot sees indications of reefs where all is a white glare to even an experienced Englishman. Consequently it is the invariable custom to get into the nearest harbour about four o’clock in the afternoon, and if the coast were not thus liberally provided, the natives’ travel by sea would be nearly impossible. The start is early next morning, between 2 a.m. and 4 a.m. according to the wind and the distance of the reefs. If the wind is fairly off shore, so that the neighbourhood of reefs will not be reached for some time, the start is early and the sail is hoisted in a strange silence, the sleepy sailors, on these occasions only, omitting their shouts and chants, and the vessel slips out of harbour like a slowly-gliding ghost.