222 
ALASKA GLACIERS 
or momentum. So, too, the roughening of the glacier 
surface where the flow is disturbed, by breaking into 
seracs and pinnacles, is analogous to, rather than homol¬ 
ogous with, the breaking of a river surface into waves. 
Viscosity causes the rupture of the ice, momentum the 
undulation of the water. The deflection of the viscous 
ice produces stresses and strains, some of which are ten¬ 
sile; in the depths of the stream the tensile stresses are 
balanced by compressive stresses due to the weight of 
overlying ice, but higher up the ice is overstrained and 
ruptured. When the swift-flowing water rises over an 
obstruction its momentum causes a portion to shoot above 
the normal level, and thus starts an undulation. 
The causes of the relation of channel depth to channel 
width are not sufficiently understood in the case of glaciers 
to warrant a comparison with rivers. The disparity of 
channel depth at the mouth of a small, fully-adjusted trib¬ 
utary is in each case a phenomenon of base-level con¬ 
trol. The surface level of the tributary is determined by 
the main stream, and if the tributary at any time erodes 
its channel rapidly, its stream becomes deeper, its velocity 
less, its power to erode less, and thus the tendency to 
deepen is limited. The resemblance of glaciers to rivers 
in this respect is a homology. 
The adjustment of channel contours to simple curves is 
brought about, in both classes of streams, by the more 
rapid erosion of projecting angles, but the work of the 
water is concentrated on these through the property of 
momentum, and the work of the ice through viscosity. 
The walled causeways sometimes built by streams of 
ice debouching onto a plain have a different process of 
construction from the similar causeways occasionally built 
by streams of water. The walls of the glacial causeway 
are made by the deposition of lateral moraines that had 
been carried chiefly as back-load. The walls of the fluvial 
