THE WORK OF SNOW AND ICE 251 



THE WORK OF GLACIERS 

 Erosion and Transportation 



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, 

 therefore, their work includes erosion, transportation, and depo- 

 sition. 



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

 accumulates on a rough surface covered with abundant rock debris, 

 the glacier already has a basal load when it begins to move, for 

 the snow covers, surrounds, 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 loose rock in its bottom, and tears off the weak 

 points of the enveloped rock-projections. It may also drag along 

 some of the earthy matter of the surface on which the ice forms, 

 and to which the ice is frozen. 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. This 

 is likely to be the case where steep cliffs tower above the snow-fields. 

 If debris descending to the glacier in this way is unburied, it is 

 superglacial, 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. This it acquires 

 partly by the rasping effect of its rock-shod bottom, and partly by 

 its power of plucking 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 the cliffs of jointed rock, but is not 

 confined to such situations. The steep bed of a valley glacier is 

 often worn more by plucking than by rasping. The advancing 

 ice gets some material, too, especially loose debris, by freezing to 

 it, for the frozen water in the soil, etc., becomes continuous with 

 the ice above, and the two move together. Superglacial material 

 may be acquired during movement, as well as before it, by the fall 

 of debris from cliffs, or by the descent of avalanches. 



