CORRASION BY GRAVITY STREAMS. 309 



and from the combination of these two facts, viz.: — channel 

 constriction and maximum depth, the corrasive force of 

 the ice sheet was relatively great at such points. In 

 ascending the range the stream lost much of its energy, 

 hence the corrasion weakened during the ascent, whereas 

 on a great descent in a channel constriction the stream 

 would gain in energy and would corrade more powerfully 

 during its progress downstream than upstream. 



In the first case the energy receives no increase, and 

 motion and corrasion are rapidly robbing the stream of its 

 energy (e.g. the Finger Lake region). In the second case 

 stream energy is increasing by the descent or the range, 

 and although corrasion is accomplished at the expense of 

 the stream action nevertheless the result is a net gain in 

 energy for the stream (e.g. the New Zealand, Norwegian, 

 Alpine and Alaskan rock basins and associated forms). 



4. The Hudson River at New York.— The main facts of 

 observation appear to be as follows : — The Hudson River 

 occupies a valley deeply sunken into the surrounding New 

 York plain, its sides being almost straight and spurless, 

 while its base lies far below sea-level. The surrounding 

 plain has been strongly scoured by an ice sheet, abundant 

 evidence of which action is supplied by the presence of the 

 roches moutonnees to be seen everywhere around New 

 York. 



From a consideration of the principles of stream action 

 it is evident that if the velocity of a stream has a decided 

 value over an area of negligible slope, then along any 

 youthful channel sunken into this area of gentle relief the 

 velocity of the stream will be much increased as compared 

 with its velocity when traversing the extra-channel (or 

 flood-plain) area. This has been shown to be true for the 

 general stream in Part I of this Series. It is simply an 

 example of material being acted on by a constant vertical 



