220 ALASKA GLACIERS 



tain gorge to an open valley, it sometimes deposits waste 

 at the sides and beneath until it comes to flow in a walled 

 causeway, or raised trough, of its own construction. It 

 may then overflow a wall of the trough and assume a new 

 course. 1 



Differences. The features of difference are equally 

 noteworthy. The speed of the glacier is very much 

 slower than that of the river, being better expressed in 

 feet per year than in feet per second. The rates of wax- 

 ing and waning are correspondingly slow. A river flood 

 is propagated downstream by the actual transfer of the 

 water added about the upper course; a glacier flood is 

 believed to be propagated downstream as a wave travel- 

 ing more rapidly than the ice. The depth and width of a 

 glacier are much larger, in relation to length, than those 

 of a river. The threads of flow in a glacier run nearly 

 parallel; in a river they weave freely in and out. That 

 which falls to the back of a glacier, though much denser 

 than the ice, does not sink to the bottom, but is carried for- 

 ward as a back-load; only light materials float on a river. 

 Most of the waste embedded in a glacier is moved along 

 continuously; most of the waste constituting the load of 

 a river is transported intermittently, being repeatedly 

 picked up and laid down. 



Homologies and Analogies. The gathering of ice into 

 streams and its downward flow are caused by gravity, just 

 as in the case of water. Most of the inequalities of veloc- 

 ity are determined by gravity in conjunction with the fric- 

 tion of the ice on the channel and the resistance of ice to 

 internal shear; and the processes are essentially the same 

 as with water. But the greater velocity on the outside 

 of a bend involves an analogy only. The bending stream 

 of ice distributes the velocities of its elements in such 



*!. C. Russell. Eighth Ann. Rept. U. S. Geol. Survey, part I, pp. 337-342, 

 360-366, 1889. 



