PHYSICAL GEOGRAPHY. 53 



For the most part the course of the Son lies in the comparatively 

 Southward bend of the unresisting rocks of the Kheinjua stage, oc- 

 Son - casionally wandering into the Rohtas area. 



Near Marai, however, it passes south of the termination of a scarp 

 of Kheinjua sandstone and of an exposure of the porcellanites to 

 flow for awhile in the soft shales of the basal stage, then turns 

 abruptly through a narrow gap in the high ridge formed by the basal 

 quartzites of the lower Vindhyans at a spot called Devalond, and 

 after a course of about 25 miles over the crystalline gneiss once 

 more breaks back through the continuation of the ridge of basal 

 quartzites just below its junction with the Banas. 



At the lower gap there is a certain amount of disturbance of the 

 Gaps in the basal quartz- quartzites which might account for the selec- 

 ite ndge. t j Qn Q f t ^\ s pos ition for the gap, but at Deva- 



lond nothing of the sort can be seen. The quartzite ridge on one 

 side is a continuation of that on the other and between them the 

 Son flows in a deep and narrow gap. This is not to be explained 

 by any diversion of drainage, and the only possible explanation of the 

 bend of the Son to the south and its double passage through one of the 

 most prominent ridges of the district, is that this course was deter- 

 mined at a time when the ridge did not exist, and that having once 

 been determined, the Son was constrained to cut its way down and 

 form the two gaps in the ridge, which itself grew in height by the more 

 rapid removal of the rock on either side. In other words, the course 

 of the Son was fixed at a time when the form of the surface was 

 determined by different conditions to those it is now dependent 

 on, and has been superimposed on the present surface features. 



The only period to which we can refer this original determina- 



Son valley marked out tion ° f the Present .course of the Son in this 

 previous to last great southern bend, is that period of reduction of 



uplift. * 



the surface to a peneplain which has been 

 referred to. At that time the relief of the land must have been 

 much less than at present and the Son must have flowed in a shallow 

 open valley, probably largely covered by alluvial deposits, and the 



( 53 ) 



