20 



Sierra Club Bulletin. 



VARIATIONS OF SIERRA GLACIERS. 



By Grove Karl Gilbert, 



OF THE UNITED STATES GEOLOGICAL SURVEY. 



Glaciers are not constant in size. If a glacier is 

 measured year after year, it is usually found to have un- 

 dergone variation, increasing or diminishing in length, 

 breadth, and thickness. The greatest changes are in the 

 length, and, as the head of a glacier occupies a practi- 

 cally constant position, the changes in length are ex- 

 pressed in the position of the lower or terminal end. 

 When Vancouver visited and mapped the coast of Alaska 

 in 1 792- 1 794, he recorded the positions of several walls 

 of ice, near which lay fleets of icebergs. He did not use 

 the word glacier, but we now know that the ice-walls 

 were the fronts of glaciers that flowed down from the 

 mountains and invaded the sea. Within the last fifteen 

 years a number of these glaciers have been remapped, and 

 it is found that important changes have taken place. In 

 Glacier Bay the ice-front has retreated about thirty-five 

 miles, and many ice-streams which were formerly 

 branches of the great trunk stream have now become 

 independent glaciers, — the Muir, the Grand Pacific, the 

 Johns Hopkins, the Reid, the Hugh Miller, etc. On the 

 other side of the Fairweather Range the variation has 

 been of opposite character, and the Brady Glacier now ex- 

 tends several miles nearer to the ocean than when Van- 

 couver saw it. Similar changes have been observed in 



