THE COASTAL PLAIN 299 



C. The Eift Valley is described as an example of an unusual and 

 peculiar physiographic type. 



Physiographic Provixces and their Eelation to geological 



Structure 



Five provinces may be rccoo-nized, extending in long, comparatively 

 narrow belts in a northerly-southerly direction. They are as follows: 

 1. The coastal plain. 2. The foot plateau, an uplifted Mesozoic coastal 

 plain. 3. The gneiss province. 4. The lava province. 5. The Eift 

 Valley. The rift lies wholly within the lava province, but it is such a 

 pronounced feature that it deserves treatment by itself and under a sepa- 

 rate head. 



The coastal Plain 



the shoreline 



The coastal plain is a narrow belt of recent coral rock, possibly of 

 Pleistocene age. The plain averages a width of 3 or 4 miles and is about 

 30 feet above mean tide, on the average. It presents a cliff frontage to 

 the sea generally and the cliffs have been eroded into fantastic forms by 

 the ocean. Wherever there are coves or in the neighborhood of streams 

 beaches occur. Generally they are small pocket types, but in some in- 

 stances they are extensive and front the shoreline continuously- for miles. 

 Below the base of the cliffs, partly exposed at low tide, there is frequently 

 a wave-cut rock terrace of varying width. 



On the whole, the coast presents a smooth outline; the irregularities 

 are due mainly to the coral reefs and to the drowning of the stream 

 deboucheres. The coast is young and wave erosion has not made serious 

 inroads upon it as yet. Outside the present shoreline, fringing and 

 barrier reefs are forming in great numbers. In general they lie parallel 

 to the coast, and some of them are so close to the surface that sand is 

 accumulating upon them; this is true even of barrier reefs several miles 

 from land. A slight elevation would extend the shoreline seaward for 

 some miles. The east coast of Africa is one of the great coral belts of 

 the world. From Somali land — with few interruptions, as off the mouth 

 of the Zambesi Eiver — to the neighborhood of Delagoa Bay, a distance 

 of 2,000 miles or more, the prevailing coast formation is coral. 



STRUCTURE AND CHARACTER 



The coastal plain is largely one of marine denudation, but in many 

 localities bordering the lagoons there occur beds of more or less con- 

 solidated derivative coralline sands, succeeded by beds of loose quartzose 

 sand and gravel, the latter derived probably from the gneiss formation 



XXI — Bull. Geol. Soc. Am.. Vol. 23, 1911 



