EXTINCT AND EXISTING GLACIERS OF COLORADO 39 



prevents the fixing of a minimum limit. An ice-body a quarter of a 

 mile long may have all the characters of the finest and largest Alpine 

 glaciers, while much larger ice-bodies may exist under peculiar cir- 

 cumstances with no glacial characters. As to the maximum limit, 

 the antarctic and arctic glaciers cover several hundred thousand 

 square miles, and the continental glacier in Pleistocene time covered 

 the greater part of the north half of North America. The width of a 

 glacier may be greater than its length, but that is unusual. 



Exaggerated ideas are common concerning the depth of glaciers. 

 Glaciation in Colorado districts usually extended only from 300 to 

 600 feet up the canyon walls, though in some places there were depths 

 of 1,500 feet, or more. None of the glaciers or neve-&elds yet reported 

 in Colorado has a probable depth of more than 100 feet, measured at 

 right angles with the plane of the surface. The minimum would be a 

 depth barely sufficient to cause motion, which depends upon slope, 

 temperature and other factors. The great Greenland and antarctic 

 ice-caps are beHeved to have a thickness of several thousand feet. 



Crevasses. — The moving ice, although brittle, will conform to its 

 bed without fracture, if conditions are just right as before explained, 

 but usually great cracks in the ice, called crevasses, open wherever 

 there is much tension, as where the center jnoves much more rapidly 

 than the edge, or where the glacier moves over an inequality in the 

 bed, when the top is stretched and gives way. Crevasses generally 

 occur in glaciers, though it is theoretically possible for one to 

 move without forming them, under favorable conditions. Still they 

 are not confined to glaciers, but occur in banks of ice and well-consoli- , 

 dated snow on steep mountain slopes, and are not conclusive evidence 

 of the glacial character of ice, as is commonly supposed. Exaggerated 

 ideas are abroad concerning the depth of crevasses. In small glaciers 

 they are Hmited by the depth of ice, and in very deep glaciers the 

 depth of crevasses is limited by the pressure of the ice. Crevasses 

 are often covered with snow, making travel over a glacier dangerous, 

 and care should always be taken when clear ice is not exposed. 



Bergschrund. — This is the great crevasse which usually stretches 

 across the ice near the walls of the cirque. It "marks the line 



