GLACIAL EROSION AS D TRANSPORTATION. 227 



two elements enter into the problem : ^1) the relative rate of 

 action of the two forces, and (2) the relative length of time 

 during which they have been in operation. 



As to time, it is evident that those have a great advantage 

 in the argument who exalt the eroding power of running 

 water. However slowly the drops may wear away the stone, 

 ample amends are made in the length of the periods through 

 which the action has continued. From the earliest ages of 

 geological history running water has been at work counter- 

 acting the effect of the forces which have elevated the con- 

 tinents. River - channels are. in fact, more constant than 

 mountain -chains. Everywhere and at all times the accmnu- 



Fn,. 71— Embossed floor of an ancient glacier in the valley of the upper Arkansas River. 



(Hayden.) 



lating waters on a continental area seek and find the lowest 

 paths to the sea. The sand and gravel which these running 

 streams push along over their beds act as the teeth of a saw 

 upon the rising mountain-summits, so that everywhere in 

 mountain -regions of great age we find deep, transverse val- 



