II. GENERAL GEOLOGY
Several articles have been published from time to time during the last 25 years dealing with some aspects of the geology of Somaliland, and those that the writer has been able to gain access to are contained in the following list. The list, however, cannot be regarded as exhaustive:
- An article by Miss C. A. Raisin, Geological Magazine, 1888, p. 414, briefly descriptive of the country from Zeyla to Mount Eilo, and that at a distance of about fourteen miles south of Bulhar, and containing an account of some rocks, metamorphic, sedimentary and igneous, obtained from these parts of the Protectorate.
- An article by Prof. J. W. Gregory, Geological Magazine, 1896, pp. 289-296, giving an account, with a section, of the geology of the central area from Berbera southwards to the top of the Golis Range, and containing a discussion on the age of the limestones.
- Another article by Prof. J. W. Gregory, Geological Magazine, pp. 26-45, giving additional information about the geology and paleontology of the Central District.
- Notes by Miss C. A. Raisin, Geological Magazine, 1903, on rocks obtained during a trek from Berbera through Harrar into Southern Abyssinia.
- An article by R. B. Newton, Quart. Journ. Geol. Soc., Vol. lxi., p. 155, on Tertiary Fossils of Somaliland.
- A resumé of the Geology of Somaliland, in “The Geology of the British Empire,” by Read.
- A Report on the Daga Shabell Oilfield (British Somaliland) by Mr. Beeby-Thompson and Dr. Ball.
- Report on the Geology of British Somaliland, by Drs. Wyllie and Smellie, of the Anglo-Persian Oil Company (with special reference to the occurrence of petroleum in the Protectorate).
A full account of the geology (including the physiography) of the Protectorate cannot be given until the examination of the numerous rock specimens and fossils collected by the writer has been completed, but as it is considered that the general conclusions arrived at as the result of the fieldwork are on the whole fairly correct, the following brief account may be given here:
(A) PHYSIOGRAPHY
Broadly considered, there are four chief elements in the physiography of the Protectorate:
(a) The Coastal Plain.
(b) The so-called Maritime Mountains and Inland Plain.
(c) The main scarp together with, as its eastern and western prolongations, the series of mountain ranges and ridges; these form the chief watershed and the most elevated portion of the country.
(d) The rolling plains on the southern side of the main watershed, which, broken here and there by isolated ridges, extend for many miles into the Abyssinian territory.
(a) The Coastal Plain
This consists of a nearly flat, barren and almost treeless strip extending from the western to the eastern frontier for varying distances from the shore. South of Zeyla in the west, the strip is at least sixty miles wide; south of Harag Jig, its width narrows to about thirty miles and, from this point to the vicinity of Berbera, it becomes progressively narrower until, behind Berbera, it extends for only seven miles to the first of the Maritime ridges. From Berbera eastwards it is in several places interrupted by limestone and gneissic ridges, which extend to the coast, and, though in places it scarcely exists at all, and in others, as behind Karam, Ankor, Raguda, widens out to several miles, in general it may be said to become gradually narrower until, near Las Gori, it is only a few hundred yards wide.
The plain is composed near the shoreline in part of beach sand, but largely of an alluvium of quartz and felspar sand intermixed with pebbles of granite, gneiss, epidiorite, and limestone brought down by the tugs from the high levels. Round Berbera, and especially at Ankor and Wagderia, it is composed, for some hundreds of yards back from the shore, of coral limestone. The present height of this limestone and the fact that at Wagderia and around Berbera and south of Elaiya occur banks of oyster shells and other recent fossils, afford interesting evidence that the coastline, at least of the central and eastern parts of Somaliland, has been rising for a considerable length of time.
There is a gradual rise in the level of the plain from west to east southwards towards the Maritime ridges, the Dubar springs on the southern edge of plain seven miles from the coast, being about 400 feet above sea level. No streams or rivers flow over it except that, occasionally after continuous heavy rain on the main escarpment, sufficient water flows in the tugs for some to escape being absorbed by the sandy alluvium and to persist to the coast. There is, however, evidence that a considerable amount of water is flowing underground to the sea, probably along very old water channels (see Water Resources below).
The plain, where not treeless, is covered in patches by low thorny khansa bushes, though, owing to the fact that the largest of these were formerly cut down by the Somalis, those that at present exist are being gradually choked by the sand.
(b) The Maritime Mountains and the Inland Plain.
These constitute the tract of hilly country which stretches from the foot of the main watershed northwards to the edge of the Coastal Plain. In reality, they consist of a plain on which are scattered numerous small steep isolated, and fragmentary escarpments and hills. The division may be said to begin with the western ends of the ridges of Bur Ad and those near Hemal in Western Somaliland and to extend eastwards as far as the west-facing scarps of Siradli and Surad Ad south of Hais. It comprises the tract lying north of Sigip and Wobleh Ranges, north of Hargeisa, and north of the Golis Range, Wagger Range, Ashararet Range, and the Sugali scarp. The numerous ridges and hills comprising it consist in part of fault blocks of Jurassic limestone and of Eocene limestone and Dubar Sandstone, with scarps facing in all directions from west through north to east, in part of ridges of granitic and hornblendic gneiss and of granite. The ridges north of Wobleh Range are scarps of Jurassic limestone overlying gneiss, succeeding one another to the north like waves of the sea, facing north-east and dipping fairly steeply to the south-west. Between Hargeisa and Bulhar and Berbera are ridges of gneiss, succeeded eastwards by blocks of Eocene limestone covered by a horizontal sheet of black basaltic lava. Daimoleh Ridge, between the Golis Range and Berbera, is a large mass of intrusive granite. Bihendula Range, between Sheikh and Berbera, consists in part of granitic and hornblendic gneisses, but in part (largely on the south side) of Jurassic limestone scarps. Al Wein Range, near the coast, Ferrio Range, and the tableland south of Ankor consist of Eocene limestone surmounted in the case of the two first by a thick deposit of gypsum. The ridges between Ankor and the base of the Siradli scarp consist of scarps of Eocene limestone. Between the various ridges are flat sandy or pebbly plains, which, with the exception of the Huguf and Las Dureh plains, are always barren. Strictly, not only is there no true Inland Plain—as the ridges and hills occur sporadically all over the area between the main watershed and the Coastal Plain—but, excluding the ridges, the rise in elevation from the coast to the foot of the main escarpment is uniform, and the only difference between the Coastal Plain and the so-called Inland Plain is the occurrence in the latter and between it and the Coastal Plain of these ridges. The average distance between the coast and the foot of the main scarp is about 40 miles and the general increase in elevation over this distance is 2,500 feet.
The flats between the Maritime Mountains are composed partly of sand from the Dubar Sandstone, partly of limestone pebbles from the various scarps, partly—as in the country between Las Dureh and Sugali Ridge —of gypsum, but largely of granitic gneiss more or less obscured by alluvium of quartz-felspar sand brought down by the tugs from the main watershed.
The Coastal Plain and the Maritime Mountains and the Inland Plain together constitute the division of Somaliland known to the natives as the “Guban” or “hot district.”
(c) The main watershed extends in a general west-to-east direction from Somadu in the southwest corner of the Protectorate to the Italian frontier south-east of Las Gori.
It is best described in the following sections:
(i) From Somadu to Hargeisa; (ii) from Hargeisa to Sheikh; (iii) from Sheikh to the Sugali Ridge; (iv) from Sugali Ridge to a point south of Hais; (v) from this point to the Italian frontier.
The watershed, and the country for a few miles to the south of it, is called by the Somalis “Ogo.”
(i) It may be held to begin with the Marmar Range south-east of Somadu, a rugged dissected mountain range over 5,000 feet in height, composed in part of granitic gneiss, in part of greenish slaty rocks which appear to be extremely sheared lavas and agglomerates, and which certainly contain masses of conglomerate with a porphyritic matrix that may be in reality a volcanic conglomerate. From this range the watershed is continued in a south-easterly direction by the great Libaheli Range, an escarpment of Jurassic limestone 6,000 feet high, resting on extremely schistose mica schists and granitic gneisses, facing north-east, with foothills composed of faulted-down blocks of the same limestone. The Libaheli Range passes into the lofty Sigip and Wobleh Ranges which trend east and west, and which consist of granitic and hornblendic gneisses with numerous intrusions of granitic pegmatite. From the east end of Wobleh Range, as far as Hargeisa, is a broken series of ridges of acid and basic gneisses, in some cases, as round Borama, capped with Jurassic limestone.
(ii) Hargeisa is situated on an undulating plateau, the height of which is from 4,500 to 5,000 feet. Behind it are scarps of Eocene limestone, and these scarps extend eastwards almost as far as Argan at the west end of Assa Range. The watershed is continued by Assa Range which, composed largely of granitic gneiss, runs in an easterly direction to Mandera at the bottom of the Jerato Pass, and the range passes directly into the Mirsa Plateau. A little to the south-east of Mandera begins the great escarpment of Eocene limestone known as the Golis Range, which, facing north, continues eastward to a point a little behind Sheikh. This escarpment has a general elevation along its entire length of nearly 6,000 feet. At the bottom of the cliff face and extending as far as Sheikh is a narrow flat plateau about a mile wide, formed largely of granitic gneiss and mica pegmatites and known as the Mirsa Plateau; its elevation is about 4,500 feet.
(iii) East of Sheikh, the Golis Range is continued by the scarps of Galgudan, the Dehemid bluffs and the Negegr Plateau, all high ridges of Eocene limestone with cliffs facing north, and with a gentle slope to the south, but, a mile or two north of the two former scarps and also forming part of the watershed, occurs the high ridge of Wagger, a mass of hornblende and granite gneiss which has an elevation of over 6,500 feet, and from the bottom of which to the north, numerous foothills jut into the Inland Plain. The rain which falls in the vicinity of Sheikh falls chiefly on Wagger Range and the Golis scarp to the west.
East of the Negegr Plateau, the watershed is continued by the serrated Ashararet Range composed also of hornblende and granite gneiss and extending in an east-west direction. At the eastern end, however, the range curves round through a semi-circle, and after running in a southerly direction for some miles, ends abruptly in Habrji Peak in the southern plain country. Ashararet Range gives place to the Eocene escarpment of the Sugali Hills which faces northeast.
(iv) Just north of the Sugali Hills, the watershed is formed of an elevated plateau of Eocene limestone, the scarps of which run in a northerly direction to a point some sixteen miles south of Hais. These scarps, which include those of Gaab, Siradli, Dud and Surad Ad, face west-south-west or west and range in elevation from 5,000 to 6,000 feet.
(v) South of Hais the main escarpment turns sharply to the east again, and continues in an east-west direction as the Afaf, Aroru and Al Hills as far as the eastern border of the Protectorate. The watershed attains its highest elevation in this section, the average height of the scarp being 6,600 feet above sea-level, while south of Mait, the limestone plateau on top of the scarp attains a height of 7,900 feet. This plateau is covered with cedar forest.
South of Zeyla in the west, the distance of the watershed or high-level ridges, i.e., Libaheli Range, from the coast is about 70 miles. Eastwards from this range, the distance becomes gradually less until, south of Berbera, the Golis scarp is about 40 miles from the coast. East of the Golis Range, the watershed represented by Ashararet Range and the Sugali Hills is again from 60 to 70 miles from the sea, but from Hais to Las Gori near the eastern border the main escarpment is never more than 16 miles in a direct line from it.
(d) On the south side of the main watershed, the whole country slopes away with a small dip for many miles, at least as far as the Abyssinian boundary.
The configuration of this part of the Protectorate is best seen from the top of the plateau south of Hargeisa, from the top of the Golis scarp, and from the high plateau north of Jid Ali in the east. Everywhere are rolling plains broken only here and there by isolated ridges chiefly of Eocene limestone, which, in contrast to those of the Inland Plain, dip at a very low angle to the south or south-east. There are no fault blocks, and with the exception of Habrji Peak, there are no igneous hills or ridges of igneous rock for many miles. Particularly is this true of the tract south and south-east of Hargeisa, in which, according to the maps at hand, there is not a hill of any size in an area of 120 miles from west to east and 60 miles from north to south. In the southeast corner of the Protectorate, it is true, there appear to be many ranges and Hills, but though they have not been examined, there is reason to believe that they mostly resemble Bur Dab Range near Kirrit, i.e., they consist of nearly horizontal Eocene limestone.
There are no rivers, and few well-defined tugs except near the summit of the watershed. The whole country is covered with reddish sand or calcareous or brownish soil, and this supports on the one hand a plentiful growth of khansa; on the other, a good growth of grass on which native ponies and sheep thrive exceedingly.
In general, looked at from some distance out to sea, the country appears to be formed of mountains and isolated ridges extending almost to the coast, and a lofty, continuous mountain range at some distance from the coast. The former are the constituents of the Maritime Mountains and Inland Plain; the latter is, in reality, partly a north-facing escarpment of the interior plateau and partly—west of the Golis Range —a continuation of this escarpment, formed of granitic and hornblendic gneisses.
The main direction of the drainage from the watershed is on the one hand northerly to the sea, on the other, southerly towards Abyssinia. On the north side of the main watershed, there are numerous tugs, some of which have cut through the face of the escarpment, and many of which have seamed the foothills at its base with ravines. Though they are now dry, there is evidence that at a former period, streams of considerable size must have flowed to the sea. On many of the fault blocks or ridges, at present streams which flow on the rare occasions when rainfalls are gradually cutting a course in a southerly direction along the dip slope. As, in some cases, a tug now flowing north has to be deflected to the west or east to circumvent the ridge, it is clear that in the course of time a passage will be cut through the ridge by the combined efforts of both streams, and the larger tug will completely capture the smaller. There are numerous instances of this having already taken place, especially in the Bihendula Range and in the Jurassic ranges in the west, large tugs now existing in the gap between two ridges that at one time obviously formed part of one and the same ridge, and from the width of the gaps and the amount and depth of alluvium that is strewn all over the Coastal and Inland Plain, there is little doubt that large streams once flowed on the surface to the sea.
On the southern slopes of the escarpment, especially north of Erigavo and Jid Ali in the east, the limestone has been extremely dissected by precipitous ravines, and excellent examples are to be seen of rounded spurs with constricted necks, which by a process of stream capture will ultimately form small round hills. In one instance, north-west of Jid Ali, the numerous south-flowing tugs have been cut off by a larger tug which runs in a north-westerly direction over the plateau, and, having cut a passage through the scarp, ultimately runs to the coast.
The rarity of tugs extending for any distance to the south on the southern slopes of the watershed is explained by the fact that the water which falls on the watershed is rapidly absorbed by the sands and sandy soil of the plains, and flows along underground channels for many miles southwards.
The country at some distance south of the watershed, and extending right into Abyssinia, in which there is little surface water, is called by the Somalis “Haud.”
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