282 GEOLOGY. 



Getting load. — If the surface on which the snow-field which is to 

 become a glacier accumulates be rough and covered with abundant 

 rock debris, as such surfaces usually are, the glacier already has a basal 

 load when its movement begins, for the snow covers, surrounds, and 

 includes such loose blocks of rock as project above the general sur- 

 face and envelops all projecting points of rock within its field. When 

 the snow becomes ice and the ice begins to move, it carries forward the 

 loose rock already imbedded in it, and tears off the weak points of the 

 enveloped rock-projections. It may perhaps also move some of the 

 soil and mantle rock of the original surface to which it is frozen. In 

 addition to the suhglacial load which the glacier thus has at the outset, 

 there may be a surface load which has fallen on the snow or ice from 

 cHffs above. This is especially true of mountain- valley glaciers. If 

 this has been buried by snow and ice it is englacial; if it Hes on the sur- 

 face it is superglacial. 



Once in movement, the ice carries away the debris to which it was 

 originally attached, and at the same time gathers new load from the 

 same area. The new load is acquired partly by the rasping effect of 

 the rock-shod ice on its bed, and partly by its rending power which, 

 under favorable circimistances, may quarry out considerable blocks 

 of rock. This '' plucking" process is at its best where the ice passes 

 over cliffs of jointed rock or steep-sloped hills. 



As the ice advances into new territory it acquires additional basal 

 load, partly by rasping, partly by plucking, partly by freezing to it, 

 and partly in other ways. One of these ways may be illustrated by 

 the sequence of events when the end of a glacier advances on a very 

 large bowlder. As the ice approaches it, the reflection of heat from 

 it melts the adjacent edge of the ice, making a slight reentrant. 

 With farther advance, the ice closes in against and around the 

 bowlder, and finally carries it along in the bottom of the moving 

 mass. In some cases, especially when its advance is rapid, the ice may 

 push debris in front of itself. Even where this is the case, the 

 amount of material pushed forward is generally slight, partly because 

 the extreme edge of the ice often fails to rest on the land in summer, 

 when the movement is greatest, being melted from below by the heat 

 of the surface over which it is spreading (see Fig. 252), and partly 

 because the earth in front of the glacier is frozen during a large part 

 of the year. In this condition, the earthy matter has greater resistance 



