THE WORK OF GLACIERS 



145 



level, the former having been roughened by the attacks of the 

 frost and other weathering agents and the latter having been 

 scoured by glaciers into rounded surfaces. Most of the lakes of 

 the high mountains, which give such scenery much of its charm, rest 

 in cirques. They lie in basins, formed either by dams of glacial 

 debris (moraines) left by glaciers, or in depressions cut into the solid 

 rock of the cirque by the ice (rock basins). An understanding of the 

 origin of cirques is, therefore, necessary for an appreciation of the 

 scenery of high mountains. 



Origin of Cirques. — If the average temperature of a mountain region is being 

 lowered as a result of a change in climate, the drifts of snow which accumulate in ra- 

 vines and spots sheltered from the full heat of the sun may last from one season to 

 the next. On account of 

 the weight of the snow and 

 for other causes (p. 142), 

 the lower layer will be 

 compressed into ice and 

 will slowly move down the 

 slope. This movement will 

 separate the moving- mass 

 of snow and ice from the 

 snow which rests upon the 

 upper slope, near the valley 

 wall. The rock wall will 

 thus be partially exposed, 

 and a crevasse, called the 

 Bergschrund (German for 

 mountain gap or fissure), 

 will be formed (Fig. 128). 

 The Bergschrund, in fact, 

 marks the line where the 



real downward motion of the neve begins. Crevasses of this sort vary in width from 

 two or three feet to more than 80 feet, and play an important part in the formation and 

 enlargement of cirques. One such Bergschrund, 150 feet deep, which extended down 

 to the rock bottom of the cirque, was explored, and its floor was found to be composed 

 of rock masses, partly or completely dislodged from the wall of the cirque. During the 

 days of summer the water from the melting snow drips down into the crevasse, wetting 

 the rocks and filling the cracks. As soon as the sun sets the temperature of such 

 regions is rapidly lowered and the water filling the cracks and joints freezes, forcing 

 the blocks from the sides. Since the cracks at the base of the rock wall are more com- 

 pletely filled with water than are those in the upper portion, the greatest disruptive 

 effect is at the bottom of the crevasse, thus tending to produce and maintain vertical 

 walls. As the cirque is enlarged by the wedge work of the ice on the rock in the 

 Bergschrund, the crevasse also moves back. The circular form of the cirque results 

 from the movement of the snow and ice away from the surrounding walls toward the 

 center of the depression. 



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Fig. 128. — The Bergschrund of a glacier. Swiss Peak, 

 British Columbia. (Photo. L. E. Westgate.) 



