Mr Ifujfli Milkr on Ricer-Terrachyi. 



299 



3. The valleys now presenting so variable a bottom, the 

 streams have been the more encouraged to work into tortuous 

 courses full of persistent bends and the amphitheatre terraces 

 to which they give rise. 



4. The restraint exercised by rock upon the modern rivers 

 strengthens their natural tendency, of which sufficient account 

 has not been made, to occupy narrower portions of valleys 



Fig. 16. 



Terraces at mouth of Nun wick Burn (North Tyne). 

 points of elevation in feet. 



Figures denote 



the more they deepen them. It has been too hastily con- 

 cluded, because rivers now occupy narrowed valleys flanked by 

 terraces comparatively broad, that therefore they have vastly 

 shrunk,- — from dimensions, in fact, proportional to the greater 

 breadth. But we must seek signs of diminution in other 

 directions than the mere extent of area occupied by rivers in 

 earlier times. The rapidity with which planation is effected 

 is obviously greatly dependent on the height of bank and 

 quality of materials. When, therefore, after the first bursts 

 following upon the glacial period, the rivers commenced to 

 work upon shallow wide-bottomed valleys, soft and yielding 

 in their nature, except where crossed by bars of rock (and as 

 such, in all cases with which we are acquainted, the glaciers 

 left our northern valleys), they proceeded to plane far and 

 wide, travelling from breadth to breadth to an extent never 

 now equalled. With banks now-a-days eight or ten times as 

 high, and rock-bound at perhaps ten times as many points, 

 it is no wonder that the modern rivers should seem to have 



