42 GEOLOGY OF THE SON VALLEY, ETC. 



ultimate consequence will be that the whole of this drainage area 

 will be captured by one or other of the two streams on either side' 

 and the gap through the ridge will remain as a dry valley or i( wind- 



gap." 



These are by no means the only instances of changes in the 



r, ,.. ,. .. e course and distribution of the drainage which 



Resulting direction of ° 



drainage channels. are to b e met w ith ; s throughout the area 

 surveyed there are numerous instances of such changes either com- 

 pleted or in progress and one of the consequences is that the stream" 

 valleys almost without exception run about east by north and west 

 by south along the strike of the softer beds, or north by west 

 across the strike of the harder beds. This distribution of the 

 drainage is most noticeable in the field and fairly well shown on the 

 map, in spite of a somewhat inadequate delineation of the hills. 



Another result has been a concentration of drainage in certain 

 tration of channels. Nothing is more certain than that a 



drainage. stream, having once been established in a given 



channel, cannot leave it to find another channel if this involves its 

 flowing uphill even a short distance. Hence we find that rivers some- 

 times traverse a narrow gorge through a hill when there may be low 

 ground on either side along which the more obvious course of the 

 valley runs. But if a tributary of the same or a different stream 

 cuts its valley back along this low ground till it intercepts the main 

 stream, this will desert its former channel and take the easier 

 course. So too if we have a series of streams crossing the strike of 

 a hard bed and if, for any reason, one of them is able to deepen its 

 channel faster than the others, then the tributary valleys of this 

 stream, cutting back along the strike of the soft beds, will intercept 

 more and more of the other streams and cause more and more of the 

 drainage on the up stream side of the hard bed to be concentrated 

 in one channel. 



An instance of this concentration of drainage is to be seen in the 

 Samdin. Below Sejari this traverses a gap 

 between a ridge of hard Bijawar quartzites on 

 ( 42 ) 



