THE WORK OF GLACIERS 



153 



usually vary from year to year and, more widely, in cycles, 1 the ends 

 of glaciers are seldom stationary for long periods. If the depth of 

 the snow in the cirque increases during a single year or a number of 

 years, the glacier will advance ; while if the snowfall is slight or the 

 average temperature high so 

 that little snow can accumu- 

 late, the glacier will retreat. 

 For example, because of the 

 hot summer of 191 1 practi- 

 cally all of the glaciers of the 

 Alps were in retreat in 191 2, 

 one (the Brenva Glacier on 

 Mt. Blanc) receding 50 me- 

 ters. The Muir Glacier in 

 Alaska has retreated seven 

 miles in the past twenty 

 years, and the glaciers of 

 the Chamonix valley in the 

 Alps, one quarter to one half 

 of a mile since 181 2. In 

 1858 there was a harbor in 

 Bell Sound, Spitzbergen, at 

 the head of which was a 

 strip of lowland and beyond 

 this a low, but broad glacier. 

 In 1 860-1 861 the glacier advanced over the lowland, filled up the 

 harbor, and extended far into the sea. It is now one of the largest 

 glaciers in Spitzbergen. 



A large glacier responds to excessive or deficient snowfall more 

 slowly than a small one, and several years may elapse before it shows 

 the effect of such changes. 



An unusual cause of rapid glacial advance is recorded from Alaska, 

 where the ice fronts of a number of glaciers have moved forward as a 

 result of earthquake shocks. During an earthquake in 1899 the 

 mountains from which the snow supply of these glaciers is derived 

 were so vigorously shaken that great avalanches of snow and rock 

 were thrown down on the neves. This increased supply caused all 

 of the glaciers in the region affected to advance. They did not all, 



Fig. 137. — The Rhone Glacier, showing 

 crevasses and front. 



1 There appears to be a climatic cycle of 35 years during which a series of cold or rainy years 

 is followed by years which are warmer or drier. 



