THE SURFACE OF A GLACIER 
I 9 7 
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 
