GLACIERS OF MOUNT RAINIER 



finds difficulty in correctly estimating the dimensions. 

 Not until an avalanche breaks from the 3OO-foot 

 neve cliff above and hurls itself over the precipice with 

 crashing thunder, does one begin to realize the depth 

 of the colossal recess. The falling snow mass is several 

 seconds in descending, and though weighing hundreds 

 of tons, seemingly floats down with the leisureliness of 

 a feather. 



These avalanches were once believed to be the authors 

 of the cirque. They were thought to have worn back 

 the head wall little by little, even as a waterfall causes 

 the cliff under it to recede. But the real manner in 

 which glacial cirques evolve is better understood to- 

 day. It is now known that cirques are produced 

 primarily by the eroding action of the ice masses em- 

 bedded in them. Slowly creeping forward, these ice 

 masses, shod as they are with debris derived from the 

 encircling cliffs, scour and scoop out their hollow sites, 

 and enlarge and deepen them by degrees. Seconding 

 this work is the rock-splitting action of water freezing 

 in the interstices of the rock walls. This process is 

 particularly effective in the great cleft at the glacier's 

 head, between ice and cliff. This abyss is periodically 

 filled with fresh snows, which freeze to the rock ; then, 

 as the glacier moves away, it tears or plucks out the 

 frost-split fragments from the wall. Thus the latter 

 is continually being undercut. The overhanging por- 

 tions fall down, as decomposition lessens their cohesion, 

 and so the entire cliff recedes. 



A glacier, accordingly, may be said, literally, to gnaw 

 headward into the mountain. But, as it does so, it 

 also attacks the cliffs that flank it, and as a consequence, 

 the depression in which it lies tends to widen and to be- 

 come semicircular in plan. In its greatest perfection 

 a glacial cirque is horseshoe-shaped in outline. The 

 Carbon Glacier's amphitheater, it will be noticed, con- 

 sists really of two twin cirques, separated by an angular 

 buttress. But this projection, which is the remnant 



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