OF THE RIFT VALLEY 
431 
The rocks along them have been removed particle by particle 
by streams or rain or wind. The material that formerly 
filled the valley has been removed as a railway contractor 
removes the material from a railway cutting. In a rift valley, 
on the other hand, the material has not been removed piece- 
meal, but the block that filled the valley has sunk between 
two series of fractures known as faults ; hence the floor 
consists of the material that originally stood at the level 
of the plateaus on either side of the valley. 
River- worn valleys and rift valleys have distinctive features. 
River-cut valleys, owing to the eddying flow of water, are 
usually sinuous : their banks run in graceful curves ; their 
sides project in spurs, and those of the opposite sides appear 
to overlap when looking up the valley. Moreover, masses of 
resistant materials remain as foothills in front of a river-cut 
bank or cliff. The views among the Kikuyu Hills, or from the 
summit of Mau westward down the valley toward Londiani and 
Lumbwa, are local examples of typical river-cut landscapes. 
A rift valley, on the other hand, is essentially trough- 
shaped : its walls are predominantly straight and even, and 
they meet the valley floor like the banks of a railway cutting. 
The walls have no spurs except where they have been notched 
by gullies, and the original scarps have been replaced by 
stream-made ravines and spurs. The cliff-like bank of a rift 
valley may cut across the country quite regardless of the 
grain of the rocks, and in this respect resembles a sea cliff 
more than a river bank ; but there is no possibility that the 
sea has eaten out the rift valley across British East Africa. 
The only available explanation of the formation of this 
valley is that its floor has sunk between more or less parallel 
fractures. 
The preparation of a section, in 1898, across the Rift Valley, 
near Lake Baringo, convinced me that only thus could its 
features and structure be explained. The parallel fractures 
as occurring south of the Kedong basin are illustrated by 
Mr. Sikes’s photograph (Fig. 1). 
The subsidence which made the Rift Valley was not a 
sudden complete collapse, like that to which tradition ascribes 
the origin of the Dead Sea and the destruction of Sodom and 
