PHYSICAL GEOGRAPHY. 55 



of equilibrium, and the only change which seems to be taking place 

 is a slow degradation of the hills ; changes of the drainage system, 

 by encroachment of one drainage area on another, are local and 

 not likely to affect the general course. In the immediate past, 

 however, there seems to have been a period when all the streams 

 were engaged in active erosion of their beds and in consequence 

 shifting of the watersheds and diversions of drainage common. To 

 the repetition of processes similar to that which has been prophesied 

 above, in the event of a period of rapid erosion setting in once 

 more, we must probably ascribe the manner in which the Son now 

 flows along the soft rocks of the Kheinjua stage. It is improbable 

 that there was only one place where the Son bent southwards from 

 the outcrop of these beds, and it is more probable that lower down 

 its course these bends have been cut off by the more rapid lowering 

 of the soft ground and diversion of drainage. By the time the re- 

 adjustment of levels had extended as far up stream as the head of 

 the east-and-west Son, the difference of level between the bed above 

 and below its southern bend had become too small to allow of the 

 diversion of this bend to be completed, and it will remain till a fresh 

 period of elevation and erosion sets in. 



From the foregoing we see that the general east-to-west course 



Son valley determined of the Son must have been determined at the 

 ^KI\Tpii£ revtou8 10 time When the S eneral surface of the ground 

 was reduced to a condition of low relief, and 

 before the last great movement of elevation. It cannot, consequently, 

 be ascribed to diversion of drainage during this period, and we see 

 that the general course of the main stream of the Son, though it 

 must be due to the greater softness of the lower Vindhyan shales, 

 and to diversion and concentration of drainage by cutting back of 

 the watersheds along this soft band, was determined some time in 

 the long ages during which this region has been exposed to subacid 

 denudation prior to the last period of upheaval and active erosion. 

 We are also able to form some estimate of the amount of the 



Amount of last great elevation which has taken place during this last 

 period, and of the height of the Kaimur scarp 



( 55 ) 



