370 



EARTH FEATURES AND THEIR MEANING 



parallel to the wall. This crack in the snow is called by its Ger- 

 man name Bergschrund or Randspalte, and may perhaps be re- 

 ferred to as the marginal crevasse 

 (Fig. 392). 



The excavation of the glacial 

 amphitheater or cirque. It has 

 been found that the marginal cre- 

 vasse plays a most important role 

 in the sculpture of mountains by 

 glaciers, for the great amphitheater 

 which is everywhere the collecting 

 basin for the nourishment of moun- 

 tain glaciers is not an inherited 

 feature, but the handiwork of the 

 ice itself. This was the discovery 

 of Mr. W. D. Johnson, an American 

 topographer and geologist, who, in 

 order to solve the problem of the 

 amphitheater allowed himself to be 

 lowered into such a crevasse upon 

 the Mount Lyell glacier of the 

 Sierra Nevadas in California. 

 Let down a distance of a hundred and fifty feet, he reached the 

 bottom of the crack, and in a drizzling rain of thaw water stood 

 upon a floor composed of rock masses in part dislodged from a wall 

 which extended some twenty feet upwards upon the cliff side of the 

 crevasse. It was evident that the warm air of the day produced 

 the thaw water which was constantly dripping and which filled 

 every crack and cranny of the rock surface. With the sinking of 

 the sun below the peaks the sudden chill, so characteristic of the 

 end of the day in high mountains, causes this water to freeze and 

 thus rend the rock along its planes of jointing. Broad and thin 

 plates of ice, loosened by melting at the walls, could be extracted 

 from the crevices of the rock as mute witnesses to the powerful 

 stresses developed by this most vigorous of weathering processes. 



In short, the rock wall above the glacier, which in its initial 

 stage was the upper wall of the niche hollowed beneath the snow- 

 drift, is first steepened and later continually both recessed and 

 deepened by an intensive frost rending which is in operation at 



FIG. 392. The marginal crevasse or 

 Bergschrund on the highest margin 

 of a glacier (after Gilbert) . 



