300 G. L. COLLIE PLATEAU OF BRITISH EAST AFRICA 



of the interior. All of the formations, whether resulting from degrada- 

 tion or aggradation, combine to form the low-lying, flat, featureless coast 

 plain, ranging from 2 to 10 miles in width. The surface of the plain is 

 covered by a thin residual soil on which a luxurious tropical forest 

 subsists. The coral rock which underlies a greater portion of the plain 

 is usually quite porous and brecciated. Eainfall sinks into it rapidly and 

 seeps away to the sea ; there are few streams as a result. 



In some cases small rivers originating in the interior cross the plain; 

 they generally have short courses and few tributaries, and have had but 

 little effect upon the topograj^hy. Their lower courses are drowned, 

 causing irregular and widely expanded estuaries, such as form Mombasa 

 and Kilindini harbors, the latter one of the great and splendid harbors 

 of the world. 



The sand and gravel beds of the coast plain are easily eroded, and are 

 well dissected by ravines a fcAv hundred yards in length. The lower 

 courses of these ravines are depressed and become the seat of delta-like 

 deposits, the tidal ^^ortion of which is covered with mangroves, while the 



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FiGTRE 1. — ficcfio)} of the Plafeaii from the Tntlian Ocean to Victoria Xyanza 



upper part, above the tide, is used by the natives for their banana sham- 

 bas. The ravine floors i^ass beneath the surface of the water. The 10- 

 fathoni contour in the lagoons is frequently notched opposite some of 

 the larger ravines, suggesting that they have a submarine extension. It 

 seems probable that the land has stood at least 60 feet higher than at 

 present within recent time. If the conjecture of Muff,- Gregory,^ and 

 others is correct that the coralline sands are Pleistocene, then the uplift 

 must have taken place late in that period or subsequently. The present 

 depression, which is the last phase in the diastrophic history of the coast, 

 must be very recent, if not now in progress. 



The Foot Plateau — its Character and Structure 



Gregory uses the term foot plateau for the narrow and relatively steep 

 front of the plateau, which faces the Indian Ocean. He limits it to that 



2H. B. Muff: Col. Rep. No. 45. Tendon, 1908. 



3 J. W. Gregory: The Great Rift Valley. London, J. Murray, 1896. 



