CORRASION BY GRAVITY STREAMS. 281 



tunities for marked increase of velocity locally and so 

 would result in relatively small examples of hanging 

 valleys. 



In the case of the old age or senile stage of land dissec- 

 tion, there cannot be : — 



1. High mountains to produce deep channels. 



2. Canon constrictions. 



3. Steep channel slopes. 



We may suppose the depth of ice traversing such locali- 

 ties to be as great as that which has worked over Alpine 

 or Fiord regions, but even then we would not get the same 

 marked interruptions of channel bases. In such a case there 

 would be no opportunity for striking local increase of ice 

 velocity to arise, with this increase of velocity again to 

 be compounded with a geometrical rise in corrasive power; 

 therefore there could be no such great corrasive results 

 in these topographies like those we find in the Norwegian 

 regions, where hanging valleys discharge over heights of 

 thousands of feet into the main channels. 1 



It would not help us in this consideration were we to 

 demand the quantitative values of the ice corrasion in each 

 case. All we are concerned with at present is the com- 

 parative result. The quantitative problem will be solved 

 later. Thus if it be demonstrated that an ice sheet can 

 corrade a plain or an area which has been dissected to the 

 senile stage — even if that corrasion be apparently feeble 

 only — then from mechanical principles it may be demon- 

 strated that in Fiord and Alpine regions a similar ice sheet 

 would produce local corrasion on such a stupendous scale 

 as to suggest to the untrained observer the action of two 

 unlike agencies in the production of the typical forms of 

 the two areas. 



1 This idea of youthful glacial Alpine dissection has been especially 

 insisted on by Davis. 



