EROSION BY GLACIERS 147 



NIK WORK OF GLACIERS 



Erosion 



Glaciers abrade the valleys through which they pass, carry for- 

 ward the material which they remove from the surface, and wear, 

 grind, and ultimately deposit it. Like other agents of gradation, 

 their work includes erosion, transportation, and deposition. 



Getting load. If the snow-field which is to become a glacier 

 accumulates on a rough surface covered with rock debris, the glacier 

 has a basal load when it begins to move, for the snow covers, sur- 

 rounds, and includes such loose blocks of rock as project above the 

 general surface, and envelops all projecting points of rock within its 

 field. When the ice begins to move, it carries forward this debris 

 in its bottom, and tears off the weak points of rock which project up 

 into it. In addition to the basal and sub-glacial load which the 

 glacier has at the outset, there may be surface debris which has 

 fallen on the snow or ice from cliffs above. If debris descending to 

 the glacier in this way is unburied, it is super glacial, but if it has been 

 buried by subsequent falls of snow, it is englacial. 



Once in movement, the ice not only moves the debris to which it 

 was originally attached, but it gathers new load, partly by the rasp- 

 ing effect of its rock-shod bottom, and partly by its power of pluck- 

 ing off or quarrying out considerable blocks of rock from its sides 

 and bottom. This plucking process is at its best where the ice passes 

 over cliffs of jointed rock, but is not confined to such situations. 

 The steep bed of a valley glacier may be worn more by plucking 

 than by rasping. The advancing ice gets some material, too, espe- 

 cially loose debris, by freezing to it, for the water in the soil freezes 

 and becomes continuous with the ice above, and moves with it. 

 Superglacial material may be acquired during movement, as well as 

 before it, by the fall of debris from cliffs, or by the descent of ava- 

 lanches. 



Conditions influencing rate of erosion, (i) Ice wears a flat 

 surface relatively little, since there is little for it to get hold of. 

 Glaciers have been known to override such a surface, burying its 

 soil and more or less of its herbaceous vegetation. Erosion is at its 

 maximum, so far as influenced by topography, when the surface is 

 rough enough to offer notable catchment for the base of the ice, but 

 not so rough as to impede its motion seriously. Other conditions 

 which influence glacial erosion are (2) the amount of loose or slightly 



