44 GEOLOGY OF THE SON VALLEY, ETC. 



exposed to subrenal denudation during which there have been 

 extensive changes of the drainage system ; and shows that the 

 present river channels may be very different from those ot the 

 drainage system previous to the last considerable change in the 

 levels of the district. 



A possible exception may be found in the Gopat, which, flowing 

 obliquely across the strike of the rocks and no- 

 where showing any special reason for following 

 its present course, except in detail, may be an ancient river which 

 has had power enough to preserve its original course and resist 

 diversion by other streams. The Son above its junction with the 

 Mahanadi crosses the lower Vindhyan area without any reference 

 to the lines of least resistance, and being a large river with a very 

 extensive drainage area to the south, may be an ancient river which 

 has not suffered diversion during the gradual lowering of the general 

 surface of the country. 



At present the determining factor in all erosion which is going 

 on is the great strike valley of the Son, which 



Valley of the Son. t „ , , t , ,.' .. . a . 



defines the base level of the tributaries flowing 

 from the south; but this itself seems to be the greatest and most 

 remarkable instance of diversion and concentration of drainage in 

 the district. The hydrography of the Son valley, as may be seen from 

 any good map of India, is peculiar. Throughout its length it receives 

 no tributary of any importance from the north, but on the south it 

 receives a number whose sources reach a distance, measured in a 

 straight line, of a hundred miles from the main stream. On the 

 north the watershed is parallel with and close to the river, and from 

 the crest of this watershed another series of rivers flow more or 

 less northward to the Ganges. The Son drainage system is conse- 

 quently a one-sided one and in this is exceptional among river 

 valleys of so large a size. 



Not less remarkable than the Son valley, which intercepts and 

 carries off the drainage of so many tributaries 



The Kaimur scarp. 



from the south, is the great Kaimur scarp, 

 ( 44 ) 



