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 



