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. 
Hoinologies 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 
1 1 . C. Russell. Eighth Ann. Rept. U. S. Geol. Survey, part i, pp. 337-342, 
360-366, 1889. 
