56 GEOLOGY OF THE SON VALLEY, ETC. 



at its commencement. The general level of the bottom of the Son 

 valley must have been at least as high as the present crest of the 

 basal quartzite ridge, which could not have existed as such when the 

 present course of the Son was determined. The highest crest of this 

 ridge now stands at 1,615 feet above the sea, or rather over 600 feet 

 above the bed of the Son ; as this peak itself must have been low- 

 ered by denudation from the height due to it from the elevation alone, 

 we may take it that the general lowering of the Son valley, which 

 cannot be widely different from the amount of elevation which has 

 taken place, must have been about 600 feet. 



As regards the height of the Kaimur scarp, this now is about 

 2,000 feet above the sea, the highest peak on the main scarp, oppo- 

 site the southerly bend of the Son being 2,310 feet, and on the Gidela 

 outlier 2,354 feet. The Kaimur range being broader and more pla- 

 teau-like than the narrow crest of the basal quartzite ridge has prob- 

 ably been lowered in a less degree by denudation, and the present 

 difference in elevation— 739 feet — is more than the difference at the 

 commencement of the last period of uplift. From this we see that 

 the Kaimur scarp must then have been in existence, but the highest 

 points probably did not rise more than 500 feet above the level of 

 the Son valley, while the depressions must have descended nearly 

 to its level. Instead of a continuous barrier there must have been 

 consequently a series of low hills with broad, open, easily traversed 

 gaps between them. 



It has been stated that the present is a period of repose 

 Recent erosion in stream an( * of little or no change in the drainage 

 channels. On the whole this is true and 

 the broad plains in the Son valley show that it must have been 

 absolutely true for a long period. The very last change in the levels 

 of the region seems, however, to have been one of elevation. In 

 the Mirzapur district the Son is bordered by a tract of lowland, 

 a river terrace, cut down to a level of about 30 feet below the general 

 level of the plain of recent deposits. Up stream the depression 

 of this terrace and its width diminish, till in Rewah territory it dis- 

 appears. In this terrace the actual bed of the Son is depressed and 

 ( 56 ) 



