THE CONTINENTAL GLACIERS OF POLAR REGIONS 275 



more rounded fragments of rock which have come up from the 

 bottom layers, and we shall again refer to the origin of such 

 moraines after the subglacial conditions have been considered. 



The grinding mill beneath the ice. If, now, we examine the 

 front of a glacier tongue which goes out from the inland ice, we 

 find that while the upper portion is white and mainly free from rock 

 debris (plate 13 A), the lower zone is of a dark color and crowded 

 with layers of pebbles and bowlders which have been planed, 

 polished, and scratched in a quite remarkable manner. The ice 

 front is itself subject to forward and retrograde migrations of short 

 period, but it is easily seen that in the main its larger movement 

 has been a retrograde one. The ground from which it has lately 

 withdrawn is generally a hard rock floor unweathered, but smooth, 

 polished, and scratched in the same manner as the bowlders which 

 are imbedded within the ice. It is perfectly apparent that the 

 latter have been derived from some portion of the rock basement 

 upon which the glacier still rests, and that floor and bowlders have 

 alike been ground smooth by mutual contact under pressure. 



This erosion beneath the ice is accomplished by two processes ; 

 namely, plucking and abrasion. Wherever the rock over which 

 the glacier moves has stood up in projecting masses and is riven 

 t>y fissure planes of any kind, the ice has found it easy to remove 

 it in larger or smaller fragments by a quarrying process described 

 as plucking. The rock may be said to be torn away in blocks which 

 are largely bounded by the preexisting fissure planes. Over rela- 

 tively even surfaces plucking has little importance, but where 

 there are noteworthy inequalities of surface upon the glacier bed, 

 those sides which are away from the oncoming ice (lee side) are 

 degraded by plucking in such a manner as sometimes to leave 

 steep and ragged fracture surfaces. The tools of the ice thus ac- 

 quired in the process of plucking are quickly frozen into the lowest 

 ice layers, and being now dragged along the floor they abrade in 

 the same manner as does a rasp or file. These tools of the ice are 

 themselves, worn away in the process and are thus given their 

 characteristic shapes. Just as the lapidary grinds the surface of 

 a jewel into facets by imbedding the gem in a matrix, first in one 

 and then in another position, each time wearing down the pro- 

 jecting irregularities through contact with the abrading surface; 

 so in like manner the rock fragment is held fast at the bottom of 



