266 EARTH FEATURES AND THEIR MEANING 



This may even continue until all parts of the upland area have 

 been buried. The snow and ice now take the form of a covering 

 cap or carapace, and the upper portions being no longer restrained 

 at the sides, now spread into a broad dome, as would a viscous 

 liquid like thick molasses when poured out upon the floor (Fig. 

 293). The lower zones of the mass and the thinner marginal 

 portions still have their motion to a greater or less extent con- 

 trolled by the irregularity of the rock floor against which they rest. 



The reverse series of changes in the glacier is inaugurated by an 

 amelioration of the climate, and here, therefore, the advancing 

 hemicycle becomes merged in the receding hemicycle of glacia- 

 tion. 



Continental and mountain glaciers contrasted. The time 

 when the rock surface becomes submerged beneath the glacier 

 is, as regards both the surface forms and the erosive work, a criti- 

 cal point of much significance ; for the ice cap and larger conti- 

 nental glacier obviously protect the rock surface from the action 

 of those chemical and mechanical processes in which the atmosphere 

 enters as chief agent, and which are collectively known as weath- 

 ering processes. Until submergence is accomplished, larger or 

 smaller portions of the rock surface project either through or 

 between the ice masses and are, therefore, exposed to direct 

 attack by the weather (see below, p. 370). 



Snow which falls in the mountains is not allowed to remain 

 long where it falls. By the first high wind it is swept off the 

 more elevated and exposed surfaces and collected under eddies in 

 any existing hollows, but especially those upon the lee slopes of 

 the range. We are to learn that glaciers carve the mountains by 

 enlarging the hollows which they find and producing great basins 

 for the collection of their snows; but with the initiation of gla- 

 ciation the inherited hollows are in most cases the unimportant 

 depressions at the heads of streams. Whatever they may be 

 and however formed, the snow first fills those hollows which are 

 sheltered from the wind, and as it accumulates and becomes 

 distributed as ice, assumes a surface of its own that is dependent 

 upon the form and the position of the basin which it occupies 

 (see Fig. 294). 



When the quantity of accumulated snow is so great that all 

 hollows of the rock surface are filled, its own surface is no longer 



