Arkell.1956.Jurassic..

CHAPTER 12
EAST AFRICA
ETHIOPIA (ABYSSINIA), ERITREA AND SOMALILAND
Our knowledge of this vast area is still patchy, but the extensive British,
Italian, German and French literature enables a general picture to be
obtained and fills in the detail at a number of scattered points. Jurassic
rocks 300-900 m. thick appear at intervals along the coastlands of the
southern end of the Red Sea and the Gulf of Aden almost from Massawa
in Eritrea to Cape Guardafui. Inland they extend under the lavas of the
high plateau of Abyssinia (Tigrai and Shoa) and crop out in a broad belt
stretching from north to south down the centre of the country, from the
Harrar province to the Juba River, there to narrow and disappear just
before reaching the coast. (Fig. 45.)
This general north-south arrangement is original, reflecting the Trans-
Erythraean trough of Jurassic times. As its appropriate name implies, the
Jurassic trough bears no directional relation to the great mid-Tertiary rift
faults of the Red Sea, the Gulf of Aden and East Africa. The faults and
rift valleys cut across the earlier seaway as if it had not existed, and it is
they and the Tertiary or late Cretaceous lavas that determine the present
major features of the topography. As shown by Gregory (1921), the fault
bounding the west side of the Red Sea branches near Massawa, the main
fork turning south along Annesley Bay and bounding the high plateau of
Abyssinia, the other, subsidiary, fork forming the Danakil horst and
continuing along the coast. Similarly the fault bounding the south coast
of the Gulf of Aden runs obliquely inland in a WSW. direction south of
Berbera, forming the northern boundary of the plateau of Harrar. About
100 miles east of Addis Abbaba the two great faults approach within
40 miles of one another, then turn off SW. to Lake Rudolf, the two fault
scarps standing face to face across the intervening rift valley. The funnelshaped
lowland between the sea and the mouth of the rift valley, forming
an equilateral triangle with sides about 500 miles long, is a sunk-land.
The western part is largely plain, the Afar of Suess. Most of the floor of
the sunk-land, like that of the rift valley, is covered by lavas, but the
underlying Mesozoic sediments are seen here and there in domes, anticlines
and subsidiary fault-blocks. In the plateaux on either side they are cut
into by erosion and appear along the river valleys.
For convenience in treating the Jurassic rocks, the high plateau of
Abyssinia north and west of the rift faults (Tigrai and Shoa) will be
described first; then the Danakil horst in Eritrea and the minor structures
in French and British Somaliland, along a course crossing the Trans-
Erythraean trough from NW. to SE. parallel to the present coast; then the
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