PHYSICAL GEOGRAPHY. 43 



the east, which had been cut off by pre-Vindhyan denudation and do 

 not extend to the west of the river, and the termination of the 

 Kharara plateau on the west. There is here a small gap of compara- 

 tively soft rocks between two ridges of hard rocks, and through this 

 the Samdin flows. 



If the course of the Samdin had been determined when the 

 surface of the land stood at a higher level than now, there would be 

 no reason why just this particular place should have been selected ; 

 but if we suppose that there were formerly several small streams 

 flowing northwards instead of one large one, and that one of these 

 flowed somewhere near the soft gap, it would cut its bed down more 

 quickly than the others, and at the same time its tributaries would 

 break into the drainage of the other streams which had not been able 

 to lower their channels so rapidly. The increase of volume conse- 

 quent on this would enable it to lower its valley still faster and, 

 extending back into the soft and easily eroded sandstones of the 

 Gondwanas, to rapidly extend its drainage area and power. 



The Banas is a similar instance. Its passage through the main 

 range of the lower Vindhyan basal conglomerate 



The Banas valley. ° _ . / & 



was doubtless determined by the gorge of the 

 Son, but the passage through the conglomerates of the red shale 

 series and lower Vindhyans near Naoria has been determined by a 

 very marked flexure in the rocks, accompanied by shearing in a 

 horizontal direction, which has lessened the power of resistance of the 

 rocks at this point. Like the Samdin we have probably a case of 

 concentration of drainage at this point, and similarly most of the chief 

 streams draining to the Son are found to have their courses across the 

 hard beds coincident with a reduction in their thickness or with some 

 structural peculiarity which locally lessens- their power of resistance. 



A coincidence of this kind is not conceivable if the courses of the 

 principal drainage channels had been determined when the surface 

 of the land formed a plain at a higher level than the tops of the 

 present hills and had then cut down along their courses to the 

 present level. It proves that the land has been for a long period 



( 43 ) 



