302 G. L. COLLIE PLATEAU OF BRITISH EAST AFRICA 



erosion has removed the softer shales and left the harder sandstone out- 

 standing as a prominent ridge, a notable object in the landscape as one 

 approaches the African coast. 



In this sandstone and grit belt the country becomes a flat plain of low 

 relief, in marked contrast to the shale belt. The region has every ap- 

 pearance of peneplanation. It has very shallow valleys, as a rule, and 

 they are occupied by intermittent streams. The valleys are wide, with 

 long, sloping sides, while at the bottom there is a stream channel made 

 during the previous rainy season. This channel is usually a steep walled 

 ditch made in alluvium, and only a few feet in width or depth. The 

 largest permanent stream observed on the foot plateau is the Mwachi, 

 with well-developed entrenched meanders 250 feet below the general level 

 of the country. The entrenching represents a period of uplift probably 

 to be correlated with that in which the basins of the coast lagoons were 

 eroded during the later Pleistocene. The entrenching is marked only 

 near the coast; a few miles inland it has disappeared; the diastrophic 

 movements which caused it must have been relatively local. On the 

 peneplained j^ortion of the foot plateau the few streams which occur are 

 almost invariably consequent upon the strong seaward slope of the pla- 

 teau. A few short subsequent streams have been developed along the 

 strike of the eastward-dipping rocks, but these as yet are of small mo- 

 ment. All of these streams have the characteristic wide valley with its 

 gently sloj^ing sides. Tlie valley sides are rarely dissected to any extent ; 

 indeed, they are hardly to be characterized as erosion slopes in the com- 

 mon acceptance of that term. They are soil-creep slopes or sheet-flood 

 erosion slopes ; that is, slopes degraded by planation processes rather than 

 by concentrated drainage. 



Much of this portion of the foot plateau is covered by a thick, almost 

 impenetrable thorny scrub; it is a desolate, uninhabited region, forsaken 

 alike by man and beast. 



The GrNEiss Province 



CHARACTER AND STRUCTURE 



Throughout the middle portion of East Africa, gneiss is the country 

 rock. It was the original massif of the continent; the total width of the 

 belt is 200 miles. It is a peneplained region, with numerous residuals 

 of gneiss. In local areas there are lava flows which proceed from fissures 

 or from mound-like volcanic cones. Generally speaking, this region, like 

 so much of central Africa, is exceedingly monotonous, and one is over- 

 whelmed not so much by the wide expanse of rolling plains as by their 

 sameness mile after mile. African topography is not, as a rule, a sue- 



