THE WORK OF GLACIERS 143 



(2) by rain and the water from the upper layers of the melting snow, 

 which soaks down into the snow, freezes, and expels the air; and 



(3) by the growth of the snow crystals. It is in this way that the 

 coarsely granular snow seen in drifts in the early spring and in the 

 neve of snow fields is produced. The growth of the crystals is 

 accomplished partly at the expense of the smaller crystals which 

 lose bulk by evaporation, while their larger neighbors take the mois- 

 ture given ofF to increase their own size, and partly from the thaw 

 water which bathes them. Neve passes insensibly into snow, on the 

 one hand, and into ice on the other. A crystallographic study shows 

 that ice is made up of crystals, the external form of which has been 

 obliterated by pressure and as a result of their growth. Ice is, there- 

 fore, a crystalline rock, like marble, and is classed as a rock. 



Snow does not accumulate indefinitely above the snow line; a 

 part melts and runs off, a part is evaporated, and a part is carried 

 away by glaciers. It has been estimated that if glaciers had ceased 

 to drain the snow fields at the beginning of the Christian era, the 

 Alps would now be buried under a mantle of snow about 5000 feet 

 thick. 



Mountain Glaciers 



Formation. — When ice has accumulated to a considerable depth 

 it tends to spread, much (so far as external appearance is concerned) 

 as does a mass of stiff molasses candy ; and if it rests on an inclined 

 surface, it tends to move down the slope. When the ice in an ice 

 field begins to move it is called a glacier. 



If we study typical glaciers, such as those in the Alps, in Glacier 

 National Park, in British Columbia, or in Alaska, we find that in 

 general they are similar but show individual differences. We find, 

 upon following a glacier to its head, that it begins in a broad amphi- 

 theater (Fig. 126), called a cirque (French for amphitheater), above 

 the snow-covered floor of which rocky walls rise precipitously, often 

 to a height of several hundred feet. In this amphitheater snow 

 gathers to great depths, often to hundreds of feet. The snow comes 

 from the frequent storms which rage there and from the accumula- 

 tions on the walls of the cirque, from which it is swept in by winds or 

 carried by avalanches. Cirques are therefore the feeding grounds of 

 mountain glaciers. In them one finds every gradation, from snow 

 which is freshly fallen, through granular neve or half-formed ice, 

 to compact ice. From the cirque the solid ice of the glacier moves 



CLELAND GEOL. — IO 



