MOVEMENT OE GLACIERS 263 



melting and refreezing of the ice crystals constituting the mass are 

 at least important ones. (See the discussion by Chamberlin and 

 Salisbury-9 :j/j-j-?j. ) Observations on glaciers flowing in valleys 

 have shown that motion is faster at the center than at the sides, 

 and near the top than at the bottom. A current analogous to that 

 found in water streams is thus produced, which also meanders more 

 strongly than does the ice stream as a whole. In broad glaciers, 

 formed by the union of a number of branches, several lines of rapid 

 motion may exist, with lines of weaker movement between. Up- 

 ward currents within the ice also exist, which bring debris from the 

 bottom to the surface at the sides. In this manner lateral moraines 

 of rock and soil are formed on the surface of the valley glacier. 

 This morainal material, augmented by that falling from the cliffs 

 between which the glacier flows, is left at the foot of the ice with 

 much that is carried along the bottom, there to constitute the termi- 

 nal moraine. 



Rate of Movement. Movement of glaciers is, as a rule, greater 

 in summer than in winter. Some of the glaciers of Greenland 

 move 50, 60, or even more feet per day in summer time, while a 

 case of 100 feet per day has been noted (Chamberlin & Salisbury-9). 

 In this case the movement in April was only 34 feet. Such rapid 

 movements occur only where the ice of large inland areas crowds 

 down into comparatively narrow fjords. In Switzerland the rate 

 of motion ranges from i or 2 inches to four feet or more per day, 

 while the Muir glacier of Alaska has been found to move 7 feet or 

 more per day (Reid). The causes affecting rate of movement 

 of the ice are: i, thickness and volume of the ice sheet; 2, slope 

 of the land surface ; 3, slope or gradient of upper surface of the ice ; 

 .4, character and relief of the rock bed ; 5, temperature ; 6, cjuantity 

 of water falling on or reaching the glacier. 



Wasting of the Glaciers. Glaciers waste away by melting at 

 their surface, front and sides, and to some extent at the bottom, 

 where heat is generated through friction and compression as well as 

 by the rise of the earth's heat through conduction under cover of a 

 thick ice sheet. Glaciers also waste away by evaporation, the ice 

 changing directly to the form of vapor. The wasting of glaciers on 

 the surface is generally spoken of as ablation. 



Erosive Work of Ice. The erosive work of ice may be twofold: 

 denudation, or the removal of material formed through decomposi- 

 tion and disintegration of the rock surface, and the active removal 

 of fresh rock material by scraping, plucking, etc., or glacial corra- 

 sion. The general process of erosion by ice has been termed cxara- 

 tion by Walther (from arare, to flow). This word might be re- 



