(B) GEOLOGY
The different rock groups of which the Protectorate is composed may be conveniently described in the following order, beginning with the oldest:
- Pleistocene and Recent Deposits.
- Lavas.
- Coastal Limestone Series.
- Post-Daban Conglomerates.
- Daban Series.
- Gypsum Series.
- Eocene Limestone Series.
- Dubar Sandstone.
- Jurassic Limestone Series.
- Inda Ad Slate Series.
- Crystalline Series (gneisses, schists, etc.).
1. THE CRYSTALLINE SERIES
The oldest rocks of the Protectorate and those on which the sedimentary series have been directly laid down are granitic and hornblendic gneisses with associated mica schists, quartz schists, foliated porphyries, granites, gabbros and pegmatites. The gneisses, in places singly but usually together, form, with the various limestones, the chief mountain ranges and ridges of the country, and, particularly in the west, they form a considerable part of the scarps even of the limestone ridges. They make up the ranges of Sigip and Wobleh; of Bur Madu north of Bur Ad; of the vicinity of Borama and the country west of it; of the large area between Borama, Hug, and Gebilay; of a large tract north of Hargeisa. They constitute the ranges of Assa north of Adadleh, the ranges around Mandera, the Mirsa plateau, Wagger Range and its foothills, Ashararet Range, Bihendula Range; part of Jirba Range, part of the country south of Hais, including the ridges of Ertoleh, Daga Har, Beyda Goyah, etc., and the range of Illagamait just west of Mait. Moreover, much of the surface of many of the plains of the Guban is composed of granitic gneiss, for example, Hussein plain, the plain near Hallimaleh, the plain south-west of Berbera, and the Galoka plain. Of the country between a line from Somadu through Borama, Hargeisa, and Sheikh to the Sugali Hills and the coast, fully half is composed of gneisses and foliated acid rocks. The granitic gneisses are pale or dark grey, in places yellow with a pinkish tinge; some are fairly, or, at least distinctly, foliated and of medium grain, others are indistinctly foliated and coarse-grained. In places, as at the west end of Libaheli Range, and just west of Fullenfuhl, near Borama, they have been so altered by pressure as to form flaky mica schists. In the grey varieties, the mica is chiefly biotite and the mica schists comprise both muscovite and biotite schists. In all these varieties, the chief felspar is microcline.
The hornblendic gneiss is usually fine-grained, granular, and rather indistinctly foliated. It forms large masses west of and around Borama, and along the course of and especially at the head of Darinwadu tug south of Buk Gigo in the Gadabursi district; it forms the range of Bur Madu just north of the Jurassic limestone ridges of Bur Ad; it constitutes many of the hills and ridges east of Besare, a large part of Wagger Range and the Sheikh Pass, of Ashararet and Bihendula Range, and of Ertoleh Ridge south of Hais. A rather curious feature of the gneiss is that, in many places, it contains masses of a coarser, apparently unfoliated and much-jointed epidiorite, which stand out usually as hills or small ridges. These are common around Borama and in the upper part of Darinwadu tug. It is not certain whether they are merely masses which have withstood the pressure that caused the foliation in the hornblendic gneiss or whether they represent dykes intruded into the gneiss. From what has been seen of the relations of these rocks, they would appear to be merely harder and more resistant portions of the gneisses, but further work on their relations is necessary. With regard to the relative ages of the two gneisses, nothing definite could be established in the time that could be devoted to an examination of the outcrops showing the junction. No evidence was found which showed which was intrusive into the other.
Observations were made throughout the tour of the Protectorate with a view to establishing the direction of regional foliation of the gneisses. Much difficulty was experienced owing to the amount of faulting in the complex and to the fact that, although accurate data of this kind are generally to be obtained only by detailed work over considerable areas, observations could, under the circumstances, only be carried out over a small outcrop in any one place. There appear, however, to be three main directions. In the west, along the axis extending from the west end of Libaheli Range through Bur Ad Range to Mandera and the Mirsa Plateau, i.e., along what may be called the central axis of the country, the strike of the foliation is about east and west (in places about 5° north of east, in places about 5° south). Around Borama and for some miles to the north, the strike of the planes ranges from N.5°E. to N.55°E. In the east of the Protectorate, from the northern slopes of Wagger, through Ashararet Range, in Bihendula Range, and in the gneissic area south of Hais (including Ertoleh, etc.), the prevailing direction is north-east and south-west.
Intruded into the granitic and hornblendic gneisses are a number of different rocks obviously of very different ages. In the neighborhood of Borama and in many other places, foliated granites, clearly intrusive, stand up as hills. Occurring us dykes in these, hut also elsewhere as dykes in the gneisses, are flesh-red and white aplites, some of which are garnetiferous. These are especially common in the west, north of Adadleh, and in the eastern part of Mirsa Plateau. In some places, there are massive, quite unfoliated dykes of grey and pink granite. Daimoleh Mountain, for instance, south of Berbera, is a large mass of intrusive granite. Inifaru Mountain, and Gunra Hill, southeast of Las Gori, though outwardly of bright red color, are also intrusive granite masses. In the country around Besare, in Western Somaliland, are large masses of extremely foliated quartz porphyries. Just east of Sheikh and between it and Wagger Range is a large intrusive mass of black norite showing fluxion banding in places. In Gardeleh tug, in the west, is a large intrusive dyke of gabbro, and to judge from the boulders in many of the western tugs there must be other intrusions of the same gabbro. Between Hemal and Buk Gigo in the west, south of Lojebir Mountain, and in other places, are intrusive masses—mostly forming the flat surface of the ground—of reddish and dark-purple quartz porphyry.
Pegmatite veins are numerous in both gneisses. In Sigip and Wobleh Ranges, Libaheli Range and in the ridges from Mandera to Sheikh, they are not only especially numerous but in many cases of great size. In Bur Madu Range, north of Daba Dulla, they occur in the hornblende gneiss in great abundance and exhibit extraordinary contortions.
In addition to the granitic and hornblendic gneisses, there occur in the Ulauleh tug, in the Gadabursi District, some quartz and graphite gneisses. Their field relations were almost wholly obscured by debris, but, as they are friable and granular, seemingly devoid of felspar, and as no similar rocks have been found forming part of the normal granitic gneisses, they would all appear to be of sedimentary origin, and younger at any rate than the granitic gneisses.
With regard to the age of the Crystalline Series, little of a definite nature can be stated. The granitic and hornblendic gneisses may be regarded as of Archaean age, and the quartz and graphite schists, if, as is probably the case, they are of sedimentary origin, may be provisionally classed as of pre-Cambrian age. The norite, the gabbro, and the flesh-red aplite dykes are certainly the youngest of the intrusions described. Obviously, however, between the formation of the gneisses and the intrusion of these dykes, a vast interval of time must have elapsed to allow of the intrusion and subsequent foliation of the foliated porphyries of the Besare neighborhood and of the foliated granite of Borama and elsewhere, and as these dykes have nowhere been seen intruding one another or any of the sedimentary formations, it is impossible to form any idea of their age.
2. INDA AD SLATES
This series, which has been recognized for the first time in Somaliland, outcrops about twenty-four miles south-east of Las Gori over an area of flat, gently undulating or low hilly country, at least twenty miles long from west to east, and five miles wide from the base of the main escarpment northwards towards the coast. It is typically developed in the neighborhood of the “durdur” and pools of Inda Ad. The rocks consist of slates, greyish and greenish quartzites, slaty sandstones, and beds of limestone. All the beds dip either vertically or at a high angle to the west (at Inda Ad), so that the whole outcrop is formed of the edges of the beds. Their prevailing strike is N. 20°—30° W. The dip at Inda Ad ranges from 70° to vertical but is chiefly from 80° to vertical in a direction a little south of west. At Kul, however, some twelve miles further east, the dip is about 40° in a direction N. 80° E., i.e., a little north of east. This change in the dip argues either a fault or a fold, but if folding has taken place, it must have been extremely acute.[5] The comparatively low angle of dip, 40°, is unusual in the series, and it appears to be only local. No attempt could be made to investigate the possible presence of such a fold or the cause of the change. The series strikes directly under the Eocene and Dubar Sandstone of the main escarpment to the west, and doubtless continues for some distance underneath it, and, as the latter is practically horizontal (it dips to the south at about 5°), a most striking uncomformity is visible from the top of the scarp looking eastwards.
The slates comprise white, grey, purple, black, striped greenish, and reddish varieties, are very fissile, and in places—as in Hedwein tug—could be used for building purposes. Between Inda Ad and Kul, they are intruded by a great number of white and reddish ferruginous quartz veins or reefs, which occur in two series, one parallel to the bedding and the other at right angles to it. The reefs range in thickness from one inch to several feet. In places they contain impressions of pyrite crystals.
Interbedded in the slates are several bands of limestone, and these bands form well-marked features in the landscape, standing up as ridges above the level of the slates to a height, in places, of 100 feet. The bands are from two to thirty feet thick, and, owing to the reddish staining on the sides and the greyish surface, they closely resemble the scarps of Eocene limestone. The dip is parallel with that of the slates. In color, on the fresh fracture, they are pale yellow, nearly black, variegated red and white, brown and brownish-red and ferruginous, and then partly striped with haematite veinlets.
Though slaty sandstones occur in the series at Inda Ad, the grey quartzites were found for the first time in Huliah tug some few miles southwest of Kul, where, with a thinly laminated structure, they exhibit, the same strike and dip as the slates at Inda Ad.
Grey quartzites also occur some three miles southwest of Jiridli, and in this locality, they are intruded by large dykes of saussuritic epidiorite. At Jiridli well, there is another large outcrop of grey quartzites which enclose fragments of the epidiorite with marked alteration borders due to assimilation. The presence of these fragments is remarkable because they are not directly connected with an epidiorite intrusion.
At Las Bar, four miles north-east of Kul, the slates appear to be intruded by masses of granite, of which the hill Inifaru is the most prominent; thin veins of the granite run parallel to the bedding planes of the slates.
The age of the Inda Ad Slate Series is unknown. A careful search was made of several large outcrops, but no fossils or traces of fossils whatever were found in any member of the series, slate, sandstone or limestone. They must, however, be considerably older than any of the other sedimentary formations, as they have been subjected to movements which have been great enough to tilt the whole series almost through a right angle, movements which were complete before any of the other sediments were laid down. Probably they are of Palaeozoic or pre-Palaeozoic age, and they may correspond to the slates of the Karagwe Series of the Transvaal.[6]
Similar slates occur some sixteen miles south-east of Hashau, up Goru tug in the vicinity of Marodi Ur, near the bottom of the scarp of the Afaf Hills. About one mile up Goru tug from where the track from Kelma meets the tug, is an outcrop of greenish, yellowish, and grey slates which dip vertically and of which the edges of the bedding-planes strike north and south. Further up the tug, at Addi, the slates are reddish, noticeably cleaved, and jointed; the bedding planes dip west at an angle of 30° and the strike of the cleavage is N. 20° E. Above Addi, are black unfossiliferous limestone bands interbedded with the slates. The series appears to pass underneath the Afaf scarp. There is little doubt that it is of the same age as the Inda Ad Slates.
Again, at the north end of Jirba Range (south of Karam), on the south side of the Bihen Gaha Pass, is an outcrop of white, grey, purple and black slates which so closely resemble those at Inda Ad that they may be included in this group. The rocks — which are exposed on the sides of two or three of the hills — have been much disturbed. They appear to be intruded by more or less felsitic quartz-porphyries and dykes of white and pink aplitic granite, and the whole mass has been extremely sheared. The strike of the planes of shearing is N. 15°—30° E., and so far as could be determined, the dip of the slates is E.N.E. and ranges from 60° to vertical. Most probably the slates lie directly on the gneisses which make up the core of the range. A noteworthy feature of the outcrop is that, intrusive into the granites and into the slates and porphyries, are thin black dykes of epidiorite which run parallel with the planes of shearing, a fact which shows that at least some of the epidiorites of the Protectorate are of later age than the Archaean and pre-Cambrian.
Another large outcrop of slates occurs some miles south of the coal deposit at Hedhed, between Godadi and Hedhed, on the track from Las Dureh to Ankor. The slates form hills and ridges which trend in a north-north-easterly direction. They are clearly sedimentary, extremely cleaved, jointed at right angles to the cleavage, and the bedding-planes are more or less parallel to the cleavage. The strike of the cleavage is N. 25° E. and the slates dip N. 65° W. at an angle of 60°. The rocks are, in different places, grey, yellow, greenish, brownish, and black in color, very fissile, in part carbonaceous, in part silicified. The Eocene limestone of the surrounding country is faulted against them. A prominent ridge of the slates runs in a northerly direction on the east side of Hodmo tug to within a few miles of Ankor.
The hills of yellow-stained marble occurring about 16 miles south of Ok Pass, known as Demirjog, and described later in the section on Marble, may belong to this Inda Ad series. In Demirjog, the marble is apparently intruded by dykes of epidiorite foliated and massive, and it has been pointed out above that in Jirba Range, dykes of a similar rock intruded the slates outcropping in the range. A patch of a similar marble was found on the track at Dorjibis in Western Somaliland,, but the field relations of this outcrop were even more obscure than those of the Demirjog marble.
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