54 GEOLOGY OF THE SON VALLEY, ETC. 



present ridge of the basal quartzites was either completely covered 

 or only represented by a few knolls of rock. In this valley the 

 river would be at liberty to wander from side to side, and at the 

 period of the last great movement of elevation it flowed in a bend 

 which crossed the outcrop of the basal quartzites in a loop not 

 unlike its present course. 



Once the movement of elevation commenced and the rivers began 

 to cut down their channels, the course of the Son was fixed, and it 

 was, where it crossed the harder rock, constrained to preserve its 

 position and form two gorges through the range of basal quartzites 

 which gradually grew in height as the ground on either side was 

 lowered at a greater rate than the crest of the ridge. In this course 

 the Son must remain till one of its tributaries, the Sohera for instance, 

 cutting back along the softer rocks of the Kheinjua stage, inter- 

 cepts the Son above Ghusa and diverts it from its present course. 



Under existing conditions there seems no prospect of this, but if 

 there were a fresh period of elevations which 



Possibility of diversion 



of the Son from its south* would set the streams at work on active erosion 

 of their beds, there can be no doubt that the 

 two barriers of hard quartzites would oppose a resistance to the 

 erosive action of the Son which would prevent its lowering its chan - 

 nel in the Kheinjua beds above Marai at the same rate as it would 

 be lowered below the junction of the Banas. This difference of 

 level would affect all the tributaries and those which joined the Son 

 below would have their gradients, and consequent power of erosion 

 increased. The Sohera would cut back its watershed and divert the 

 waters of the Nagour above Deora into its channel, after which a 

 tributary of this river would cut back along the strike of the shales 

 south-westwards of Deora till the watershed was cut back to the Son, 

 and the waters of that river diverted into a new and easier course. 



As already indicated there are no signs of the probability of 

 Present a period of com- such diversion taking place under present con- 



parative repose. ditions. The present appears to be a period of 



comparative repose, the streams have nearly attained a condition 

 ( 54 ) 



