53 GEOLOGY OF THE SON VALLEY, ETC. 



valley, that is to say, to the determination of whether this course 

 was determined prior to or consequent on the last great uplift. 

 This resolves itself into the possibility of determining whether there 

 is any trace of drainage channels having crossed the Kaimur range 

 from south to north. 



Of this there is certainly no satisfactory evidence. The height of 

 No gaps through the the crest of the scarp varies greatly and there 

 Kaunur scarp. are com p ara ti ve ly low gaps which have been 



selected as passes across the range, but they are all broad and open 

 where they cross the crest, and there is no narrow notch such as 

 would be cut by a river during a period of erosion. It must be re- 

 membered that if the drainage had been mainly northwards previous 

 to the last great upheaval, and the Son had gradually cut its valley 

 backwards diverting more and more of the drainage from the south, the 

 cross valleys to the east might have had very little time to lower 

 their beds in the upper Vindhyan area before they were diverted. 

 Those further west, however, would have had more time, and one 

 would expect to find at least a somewhat deep-cut valley mark- 

 ing the point where what is now the upper Son — above its junction 

 with the Mahanadi— crossed the Kaimur outcrop to the low ground 

 further north. No such low gap can be found, and, though there 

 has been a certain amount of concentration of drainage since the 

 Son last began to lower its valley, and the streams of a former period 

 may have been more numerous and smaller than at present, it is 

 difficult to account for the absence of such gaps, if the Son valley is 

 of recent origin. 



It will be seen from the foregoing that we have on the one hand 

 the absence of tributaries from the north pointing to a recent 

 origin of the present valley of the Son, and on the other the absence 

 of wind-gaps in the Kaimur scarp pointing to its antiquity. Had 

 we nothing else to depend on it would be difficult to choose between 

 the two, but there is fortunately one peculiar feature in the course 

 of the Son which affords what appears to be decisive evidence. 

 ( 52 ) 



