THE SURFACE OF A GLACIER 197 



This leveling of the surface is of course ultimately 

 caused by gravity, and it is immediately brought about by 

 variations in the flow of the ice, the directions and veloci- 

 ties of the flow of different parts being so modified as to 

 permit the ice to pass around and over obstructions with- 

 out lifting the glacier bodily. The existence of internal 

 variations in direction, being attested by the arrangement 

 of striae on lands that have been glaciated, is a well-recog- 

 nized fact, but variations of direction alone are not suffi- 

 cient to explain the coexistence of an approximately even 

 glacier surface with a very uneven glacier bed. The con- 

 dition of continuity can not be satisfied without variations 

 of velocity also within the mass. The adjustment of an 

 ice stream to an irregular channel through the formation 

 of differential currents is thus quite analogous to the ad- 

 justment observed in water streams, although the greater 

 viscosity of the ice may be assumed to prevent the occur- 

 rence of reversed currents or eddies. 



Another factor in the leveling of the ice surface is prob- 

 ably connected with ablation, or the process of wasting by 

 melting and evaporation. Portions of the Muir and Hugh 

 Miller glaciers which were demonstrably stagnant, never- 

 theless exhibited to the general view a conspicuous even- 

 ness, and the same remark applies to glacier remnants 

 stranded, like Reid's ' Dying Glacier,' on saddles, or trough 

 summits. These motionless and wasting ice bodies, 

 though not tabular but curved downward toward their 

 edges, were bounded by simple symmetric contours, in- 

 dicative of equable reduction by the wasting agents and a 

 general interdependence of process for the whole surface. 



Lateral Cliff. Coexistent with the general tendency 

 toward evenness are several kinds of unevenness, each 

 determined by some evident special condition. Wherever 

 the side of an ice stream was observed adjacent to a rock 

 wall, it was found to present a cliff toward the rock, the 



