Encyclopaedia Britannica, 11th Edition, "Bréquigny, Louis Georges Oudard Feudrix de" to "Bulgaria" / Volume 4, Part 3

by Various

The rift-valley, though with a generally level floor, is divided by transverse ridges into a series of basins, each containing a lake without outlet. The southernmost section within British East Africa is formed by the arid Dogilani plains, drained south towards German territory. At their north end rise the extinct volcanoes of Suswa (7800 ft.) and Longonot (8700), the latter on the ridge dividing off the next basin—that of Lake Naivasha. This is a small fresh-water lake, 6135 ft. above the sea, measuring some 13 m each way. Its basin is closed to the north by the ridge of Mount Buru, beyond which is the basin of the

still smaller Lakes Nakuro (5845 ft.) and Elmenteita (5860 ft.), followed in turn by that of Lakes Hannington and Baringo (q.v.). Beyond Baringo the valley is drained north into Lake Sugota, in 2° N., some 35 m. long, while north of this lies the much larger Lake Rudolf (q.v.), the valley becoming here somewhat less defined.

On the west of the rift-valley the wall of cliffs is best marked between the equator and 1° S., where it is known as the Mau Escarpment, and about 1° N., where the Elgeyo Escarpment falls to a longitudinal valley separated from Lake Baringo by the ridge of Kamasia. Opposite Lake Naivasha the Mau Escarpment is over 8000 ft. high. Its crest is covered with a vast forest. To the south the woods become more open, and the plateau falls to an open country drained towards the Dogilani plains. On the west the cultivated districts of Sotik and Lumbwa, broken by wooded heights, fall towards Victoria Nyanza. The Mau plateau reaches a height of 9000 ft. on the equator, north of which is the somewhat lower Nandi country, well watered and partly forested. In the treeless plateau of Uasin Gishu, west of Elgeyo, the land again rises to a height of over 8000 ft., and to the west of this is the great mountain mass of Elgon (q.v.). East of Lake Rudolf and south of Lake Stefanie is a large waterless steppe, mainly volcanic in character, from which rise mountain ranges. The highest peak is Mount Kanjora, 6900 ft. high. South of this arid region, strewn with great lava stones, are the Rendile uplands, affording pasturage for thousands of camels. Running north-west and south-east between Lake Stefanie and the Daua tributary of the Juba is a mountain range with a steep escarpment towards the south. It is known as the Goro Escarpment, and at its eastern end it forms the boundary between the protectorate and Abyssinia. South-east of it the country is largely level bush covered plain, mainly waterless.

[Geology.—The geological formations of British East Africa occur in four regions possessing distinct physiographical features. The coast plain, narrow in the south and rising somewhat steeply, consists of recent rocks. The foot plateau which succeeds is composed of sedimentary rocks dating from Trias to Jurassic. The ancient plateau commencing at Taru extends to the borders of Kikuyu and is composed of ancient crystalline rocks on which immense quantities of volcanic rocks—post-Jurassic to Recent—have accumulated to form the volcanic plateau of Central East Africa.

The formations recognized are given in the following table:—

Sedimentary.
Recent		1. Alluvium and superficial sands.
		2. Modern lake deposits, living coral rock.
		3. Raised coral rock, conglomerate of Mombasa Island.
Pleistocene		4. Gravels with flint implements.
		5. Glacial beds of Kenya
Jurassic		6. Shales and limestones of Changamwe.
Karroo		7. Flags and sandstones.
		8. Grits and shales of Masara and Taru.
Carboniferous?		9. Shales of the Sabaki river.
Archaean		10. Schists and quartzites of Nandi.
		11. Gneisses, schists, granites.
Igneous and Volcanic.
Recent		Active, dormant and extinct volcanoes.
Post-Jurassic to Pleistocene		Kibo and volcanoes of the rift-valley.
		Kimawenzi, Kenya and plateau eruptions.

Archaean.—These rocks prevail in the districts of Taru, Nandi and throughout Ukamba. A course gneiss is the predominant rock, but is associated with garnetiferous mica-schists and much intrusive granite. Hornblende schists and beds of metamorphic limestone are rare. Cherty quartzites interbedded with mylonites occur on the flanks of the Nandi hills, but their age is not known.

Carboniferous?—From shales on the Sabaki river Dr Gregory obtained fish-scales and specimens of Palaeanodonta Fischeri.

Karroo.—The grits of Masara, near Rabai mission station and Mombasa, have yielded specimens of Glossopteris browniana var. indica, thus indicating their Karroo age.

Jurassic.—Shales and limestones of this age are well seen along the railway near Changamwe. They contain gigantic ammonites. According to Dr Waagen the ammonites show a striking analogy to forms from the Acanthicus zone of East India. Belemnites are plentiful.