62 MIDDLEMISS: PHYSICAL GEOLOGY OF SUB-HIMALAYA. 



elevation of the rock into an inclined position of 40° has given 

 no time to the river to meander and widen out its bed. With it's 

 action sharpened by resistance, the river has had to cut as it were 

 against time, else the barrier of rock would have risen against it and 

 driven it to find an exit by some other track, just as it had already 

 been turned far out of its direct course to the plains by the rising 

 barrier of strata further east, alluded to in Chapter II. But in the 

 dun, the flatter arrangement of the beds marks a less energetic eleva- 

 tion of them : there was less necessity for the river to cut a vertical 

 track, and so it had leisure to wander capriciously, changing its bed 

 again and again, and spending horizontally the energy that lower 

 down was acting vertically. It is necessary to bear in mind this law 

 of river action, when contemplating the formation of, duns ; for, though 

 their main features and boundaries are exclusively due to the original 

 moulding of the earth's crust by a lateral compression, the whole is 

 subsequently softened, and brought to a more general level by the 

 secondary action of the river in changing its bed and by reason of its 

 sinuous course. 



The plainward zone of Nahans begins as fairly hard, yellow sand- 

 stones and shales of the ordinary type and with concretions in their 

 upper layers. The dip is 30 to 45 N.E., increasing in steepness as 

 we reach the first important bend in the river. As we still ascend, 

 following the forest road to Boksa*r, the sandstone gradually becomes 

 less hard and merges imperceptibly into the sand-rock stage. About 

 if miles above Kalagarh the sand-rock sets in with certainty. There 

 is no change of dip, which continues N.E. and N.N.E. at from 38 

 to 40 . At the junction with the Sona N. the rock has become 

 thoroughly of the sand-rock type, so soft and friable as to be un- 

 mistakeable. The orography in this neighbourhood, especially near 

 the boundary between the sand-rock and the Nahans, is remarkable 

 for the great prevalence of the triangular wedges of strata which form 

 the hill spurs. They rise one behind the other, displaying in plan 

 a set of contours as shown in fig. 1, and in profile as shewn in 

 fig. 2. 



( 120 ) 



