(C) FAULTING AND SEQUENCE OF GEOLOGICAL EVENTS
It is generally believed[11] that in pre-Jurassic times, Somaliland formed part of a continent that was continuous with South Arabia and with India. When this continent began to break down in the Jurassic age, the movements associated with the breakdown in all probability produced similar results in Somaliland to those produced in other parts of the continent. Several marked features of the main watershed, e.g., Wagger Range and Ashararet Range, doubtless date from this period. Moreover, the tremendous movement, probably folding, which brought about the up-ending or tilting of the Inda Ad slates so that their bedding-planes are at present almost vertical over a wide area, took place certainly before the main series of faults which have affected all the formations from the Jurassic onwards on the north side of the watershed. It is, perhaps, of pre-Jurassic age. North of the main watershed or escarpment, the whole of the Protectorate has been subjected to extreme and long-continued faulting movements that produced the great system of rift valleys extending from the Sabi River south of Lake Nyassa to Aden, and diverging from Aden north-eastwards as the Gulf of Aden, and north-westwards as the Red Sea. This faulting began in post-Eocene times, probably in the Oligocene age when the movements began which ultimately gave rise to the Red Sea, and continued until the beginning of the Pleistocene and Recent age, for every member of the sedimentary series in Somaliland from the Jurassic to the Coastal Limestone has been faulted to a greater or less degree. Moreover, the faulting has not been confined to a series of parallel fractures. There appear, indeed, to be several different systems, but their relations are so complicated that only a detailed study of the tectonics of the Protectorate will enable them to be completely unraveled. So far, four main systems have been recognized:
(a) That trending in a direction north-west-south-east. This system is well shown in Western Somaliland, where it has determined the successive parallel scarps of Jurassic limestone forming the ranges of Libaheli, Bur Ad, Karimo, Aga Sur, Eilo, etc., ranges which trend in a direction north-west-south-east (bearing about 310°-130°), and, of course, face north-east. Moreover, as an examination of the map of Somaliland shows that the coast from Bulhar to Zeyla runs in this same direction, it is highly probable that the same system of faults has determined the direction of this part of the coast.
(b) That trending in a direction approximately east-west. This system has doubtless been responsible for the east-west trend of the Golis scarp, and the face of its easterly and westerly gneissic extensions, and for the east-west direction of the coast from Bulhar to Berbera.
(c) That following a direction north-south. This system has caused the abrupt termination or cutting-off of the Ranges of Western Somaliland, e.g., Sigip, Bur Ad, Bur Madu, etc., and has perhaps determined the western face of the scarp of Siradli, Dud, and Surad Ad, and the low-lying nature of the country at the foot of these scarps.
It may also be responsible for the scarp of the Manna Hills, south-east of Las Gori, which runs in a north and south direction and faces east.
(d) That running in a direction east-north-east-west-south-west. According to Wyllie and Smellie, this system determines the north-western face of the Harrar Plateau in Abyssinia, to the southwest of Borama. It may have caused the east-north-east trend of the coast from Raguda to Mait.
The earliest surface of the Protectorate was formed of granitic and hornblendic gneisses of Archaean age and some quartz and graphitic schists probably of pre-Cambrian age. At some period in the Palaeozoic area, there was an invasion of the sea over the eastern part of the country, and the series of slates and limestones of Inda Ad and Goru tug were laid down. As the result of great earth-movements either of pre-Jurassic age or connected with the breakdown of the Gondwana continent, these slates were tilted nearly vertically and intruded by granite. The breaking-down of Gondwanaland followed, and there was an outpouring of basalt lava which is represented by the basalt at the base of the Jurassic Series in Bihendula. At the same time, the western and south-western part of the Protectorate was invaded by a sea which had its greatest extent and depth in Abyssinia and in which Jurassic sediments were laid down. A small branch of this sea extended eastwards as far as Ambal and Karam. At the close of the Jurassic, the sea shallowed and finally disappeared in Somaliland, and, with the advent of the Cretaceous, enormous deposits of sand were formed on the Jurassic sediments and on the surface of part of the present plateau (on the gneisses of the Golis, of the ridges behind Ertoleh, and near Hargeisa). Erosion of the sandstones then took place to a greater or less extent and was followed by an Eocene marine transgression which was deepest in the east and south-east and gradually shallowed out in the vicinity of Hargeisa. In time, this Eocene sea retreated, leaving large lagoons or inland seas in which great quantities of gypsum and anhydrite, gypseous shales, and some celestite were formed. At the close of the Eocene or during the Oligocene, extensive faulting took place, causing the downthrow of each member of the sedimentary series beginning with the Jurassic, and the ultimate trend of the coast of the Protectorate. This faulting did not take place in one particular period, but occurred at intervals and affected the formation of each age in turn down to the Coastal Limestone. As the area north of the main watershed subsided, thick sandstones and clays were formed in a basin south of the Al Wein Range — the Daban Series — and the faulting which continued after their deposition produced in them a dip to the south, corresponding with the dip of the Gypsum and Eocene strata. This Daban Series was then covered with limestone-and-gneiss conglomerate produced by erosion of the earlier formations. In the later stages of the history of the country, coral reefs which had formed on the shores were elevated by a rise in the level of the coast, and faulted just before the long-continued faulting ceased. As is shown by the presence of coral fringes, shell-banks, and coral platforms some distance from the beaches, the coast is still rising.
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