Fig. 79. The maritime plain and Irba Mountains
Yemêna oasis in foreground and gravel-covered ridge across middle
distance
Plate XXXVII
Figs. 80 and 81. Corals, in position of growth, on the summit of Jebel Têtâwib
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
Fig. 82. Water-hole under a stratum of hard gravel and gypsum conglomerate, in the lower part of the ravine
Fig. 83. Exposure of sandstone under the gravel of the
plain
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.
Fig. 84. Approaches to Port Sudan, shewing Sanganeb atoll, portions of the barrier reefs and fringing reef. The latter dotted.
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
Diagram 9. Section through Rawaya and Makawar.
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.
Fig. 85. Peninsula of Rawaya and reef systems connected with it
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.
Diagram 10. Conversion of a line of low hills, partly submarine, into a maze of surface reefs
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
Figs. 86 and 87. Two views on Rawaya
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.
Diagram 11. Fringing and barrier reefs in neighbourhood of Ras Salak, shewing correspondence of both systems with coral formations on land
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
Fig. 88. In a fault ravine of Abu Shagara. Cliff coral, gypsum and sandstone, the latter containing sheets of recrystallised gypsum, selenite, in every crack
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.
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
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.
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.
Fig. 90. Two of the canal-like “Khor” which run into the coral plain. Soundings in fathoms, note comparatively great depth of the water
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
Fig. 91
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.
As already remarked the formation we have described is that of almost the whole Red Sea, but south of Suakin it loses the regularity that is so noticeable northwards. The area marked on the map (p. 126) as “Suakin Archipelago” consists of innumerable small reefs, shoals and islets, the water between being of very irregular depth, three hundred fathoms being found close alongside surface reefs. Opposite Trinkitat the water shoals gradually, only 30 to 40 fathoms being found as far as 45 miles from land. The shore itself is extremely low and sandstone hills are absent. The area marked Shubuk, south of Suakin, shewn on the map enclosed by a thin semicircular line on the north and east sides and by the land on the west and south, has a very remarkable structure. An area of 100 square miles is enclosed by a regular and unbroken reef, indicated by the above curved line, the space within being a most intricate maze of reefs with comparatively deep canal-like passages between them. In the south are broader passages and some islets of elevated coral. The bounding reef is extremely regular and, on its eastern side, unbroken. It consists of a steep slope and precipice of growing coral up to near the surface, when the slope becomes very gradual and forms a nearly smooth surface of stunted corals with grey Xenia. This extends to among the breakers, above which is a band of gravel formed of broken and wave-rounded pieces of coral. Within is sand, and the coral capped sandbanks of the labyrinth. Landwards the ground is for miles so low and so much broken into by salt lagoons and marshes that a definite coast-line can scarcely be said to exist.
This is the remains of an old delta of the Khor Baraka, a river which rises in the Abyssinian highlands, but which nowadays never reaches the sea. Its floods come down on to the maritime plain at Tokar, where, spreading over a considerable area, they render possible the growth of cotton and other crops in the fine soil they leave soaked with water. Tokar is thus the one fruitful spot of any size on the whole Red Sea coast, but its character of fertile oasis is but short-lived. When the crops are gathered even the dry cotton stalks must be removed lest they should collect the sand, which, every day of the summer, is carried over them by burning gales, and so would convert the fertile ground into barren sandhills.
As it is, at any rate at present, impossible to predict these floods the seed must be sown after each, even though it frequently happens that another flood comes down and carries it all away. The seed must then be patiently re-sown, and that left by the last flood will grow and bear. In the old days, before man was there to make any use of it, the Baraka formed a regular delta, subject to yearly floods, a miniature Egypt. The growth of coral in this neighbourhood would then be impossible, the shifting sand, muddy and freshened water rendering its life impossible.
Now when the rainfall decreased so that for the greater part of the year no freshwater stream entered the sea, and the materials of the edges of the delta became stationary, coral growth arose here and there, forming a fringing reef, the extension of which seawards must have been exceptionally rapid in this gradually shelving water. There was then an elevation of the sea-bottom, here comparatively slight, and the sea began to cut into the raised coral, carving it out into islets, surface reefs, tidal channels and lagoons.
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.
(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.
(3) There have been three successive systems of barrier reefs along the Red Sea coast, which by continual uplift have become,
| (a) | A range of sandstone hills rising from the alluvial maritime plain. |
| (b) | A fringe of limestone along the present coast-line. |
| (c) | The present barrier system. |
(4) These three ridges were formed by the faulting of sedimentary rocks which overlay the bases of the Archean hills at the time of the great movement which opened the Red Sea section of the Great Rift Valley.
(5) The northern ends of several sections of the present barrier reefs are elevated above sea-level, and examination of these, and of the hills of the maritime plain, enable us to reach the above conclusions.
(6) At the same time Rawaya gives evidence of a seaward movement as well as uplift, Khor Dongonab and some at least of the channel within the barrier reefs being recent fault depressions, not merely an anticlinal fold formed at the opening of the Rift Valley.
The harbours and other fissures in the coral limestones, &c. of both coast-land and of the barrier reefs are due to the same secondary faulting.
(7) The maritime plain had its maximum seaward extension after the growth of coral on the second and third barriers. Owing to elevation nothing has been added to its seaward slopes since the formation of the features of the present coast-line by secondary faulting.
(8) The filling in of valleys and the completion of the connection of the second barrier with the maritime plain has been largely due to blown sand. The process is continuing, e.g. an extensive plain near Dongonab shews perfect uniformity in its formation.