45 GEOLOGY OF THE SON VALLEY, ETC. 



unmistakable indication of any considerable stream having flowed 

 in this direction ; there is no deeply-cut wind-gap to show where 

 a river once flowed, before it had been intercepted by the growth of 

 a valley cutting backwards along the strike. 



This peculiar orography and hydrography can only be explained 

 by a consideration of the geological history of this region and itself 

 seems to throw light On the vicissitudes it has undergone during the 

 long period it has been exposed to subaerial denudation. 



In the Vindhyan period, and at the time when the mountain 

 Original course of chain still existed, the course of the drainage 

 must have been northwards from the mountains 

 across the plain of deposition in which the Vindhyan system was 

 being deposited. By the time the deposition of the Gondwanas 

 commenced, this mountain range had subsided, and as the low ground 

 and the area of deposition lay to the south, we may conclude 

 that the drainage was in that direction ; we cannot, consequently, 

 look for any trace of the original Vindhyan drainage in that of the 

 present day as it had been obliterated and replaced by a different 

 system even in Gondwana times. 



After the close of the Gondwana epoch, a time corresponding to 

 Long period of subaerial the close of the Jurassic period, we have no 

 drainage. direct geological evidence of what occurred. We 



know that neither this nor any other part of the peninsula has been 

 under the sea since a much earlier period of geological history, but 

 after the close of the upper Gondwana period no great changes 

 seem to have taken place till the great outburst of the Deccan trap. 

 It seems certain at any rate that any changes which may have 

 taken place did not lead to the accumulation of deposits sufficiently 

 extensive for them to have been preserved till the present day. 



What happened during this long lapse of time there is no 



Reduction of surface direct means of knowing, but there are not 



relief * wanting indications that the country was 



reduced to the condition of low relief, which has come to be known 



as a peneplain. If the trigonometrically determined heights of the 



( 46 ) 



