extinct and existing glaciers of colorado 37 



Phenomena of Existing Glaciers 



Certain characters are possessed by all glaciers and others by most 

 glaciers, but all of these characters may under certain circumstances 

 be exhibited by ice-bodies which are not fairly entitled to the name. 

 Among them may be mentioned the following : 



Movement. — Although brittle, the ice of a glacier moves outward 

 or downward from the area of accumulation, not by the sliding of the 

 mass as a whole, but in a manner not yet thoroughly understood, 

 though somewhat analogous to the very slow flow of a viscous mass, 

 conforming to the shape of the bed over which, or the channel within 

 which, it moves. Both the observation of glaciers and laboratory 

 experiments show that ice may be "moulded into almost any desired 

 shape if carefully subjected to sufficient pressure, steadily applied 

 through long intervals of time."^ 



The rate of movement varies from a few feet to hundreds of yards 

 per annum, and is greatest in summer. It is also usually greatest near 

 the center, bending the lines of stratification into segments of circles. 

 Similar movement is frequently found in steep snow- and ice-banks 

 which are not glaciers, and quite certainly the movement of a glacier 

 is initiated in the nem itself, or the ntoe, would not continue to renew 

 the glacier by passing down into it. Therefore, the fact of movement 

 simulating that of a glacier does not prove that a given ice-body is a 

 glacier. The movement of the glacier gives rise to "rock-flour" 

 sediments, crevasses, moraines, etc. 



Moraines. — Earth, rocks and other debris are picked up by freez- 

 ing to the edges and bottom of the ice, by falling from adjacent cHffs, 

 and by being blown or washed upon the surface from near-by exposed 

 land areas. This debris is carried forward and deposited where the 

 ice reaches an altitude (in case of mountain glaciers) at which it melts. 

 Thus terminal moraines are formed at the extremity, lateral moraines 

 along the sides and ground moraines along the bed of the ice-stream. 

 In case of mountain glaciers which are furnished with abundant rock 

 debris from the crumbling of adjacent canyon walls, terminal moraines 

 are often built very rapidly, so that if the end of the ice-tongue remains 



» Chamberlin, Thomas C, and Salisbury, Rollin D., Geology, Vol. I, pp. 248-49, 1905. 



