Arkell.1956.Jurassic..

3°4
EAST AFRICA
main central outcrops on the south and east of the rift faults, from the
Harrar plateau southwards through Jubaland to the Indian Ocean.
THE NORTH ABYSSINIAN PLATEAU (TIGRAI AND SHOA)
The foundations of our knowledge of the geology of this region were
laid by W. T. Blanford (1870), of the Geological Survey of India, who
was deputed by the Government of India to accompany as naturalist the
British Army on its march from Annesley Bay in Eritrea to Magdala and
back in 1868. His clear and fascinating account is well worth reading
still. The expedition marched due south along the western edge of the
plateau by way of Adigrat and Antalo to Magdala.
Resting upon the metamorphic basement complex is up to 300 m.
or more of unfossiliferous sandstone, which Blanford named the Adigrat
Sandstone. 'It is usually white or brown in colour, the former much
predominating. Occasionally it is pale brown and lilac, or, as about
Takonda, brick red; these colours being chiefly restricted, however, to
bands interstratified with the mass of the rock. It is usually very quartzose,
frequently felspathic, less commonly argillaceous. Shales of a blue or
lilac colour are frequently met with towards the base of the group, but
the principal characteristic of the great bulk of the sandstone is its massive
character and the absence of marked bedding, so that the high cliffs
of it which form the head of the Haddas and Komayli ravines, and which
surround the valleys south of Senafe, appear as if cut out of a huge
unstratified block'. (Blanford, 1870, pp. 170-1).
Above the sandstone, with a conformable junction, follows the Antalo
Limestone. 'It is usually in thin beds of grey colour, less commonly
ochreous, and it much resembles some of the beds of Lias limestone in the
south-west of England. The rock when broken is compact and earthy,
or but slightly crystalline.' (Blanford, p. 176.) Generally the traps of the
plateau summit overspread the Antalo Limestone directly, but at intervals
a higher sandstone is intercalated, representing outliers that existed before
the lavas were erupted. This sandstone is also unfossiliferous, but it
now seems likely that it is Lower Cretaceous and approximately to be
equated with the Nubian Sandstone.
Blanford recorded a list of mollusca and two echinoids from the Antalo
Limestone, but unfortunately no ammonites were found. His records,
and some figures, suggest at least two horizons: Bathonian [Trigonia
pullus Sow., Modiolus imbricatus Sow.), and Upper Jurassic, probably
Lower Kimeridgian.
Further progress has been made to the south, in Shoa (Choa), north
and NW. of Addis Abbaba. Here tributaries run down to the Blue
Nile (Abai) in tremendous gorges, the sides of which lay bare all the strata
of the plateau. The Jurassics reach a height of 8500 ft. above sea-level.
The succession worked out by Aubry (1886), H. Douville (1886) and
Futterer (1897) is as follows (the classification in stages being as usual the
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