CHAPTER V 

 THE WORK OF GLACIERS 



When viewed from an eminence, a mountain glacier has the appear- 

 ance of a river of ice flowing down a valley to a point where it ends 

 abruptly and a stream emerges from beneath it and courses toward the 

 sea. If the climate is cold, as in Greenland, glaciers may even reach 

 the sea, where their shattered fronts are carried away as icebergs by 

 the ocean currents. 



General Considerations 



Distribution and Size of Glaciers. — Glaciers exist on high moun- 

 tains, even in the tropics. In temperate regions they abound on high 

 ranges, especially on those against which moisture-laden winds blow; 

 as, for example, the Sierra Nevada, the Cascade ranges, the Alps, 

 the Caucasus, the Andes, and the Himalayas. 



Mountain glaciers vary in size from those which barely extend 

 beyond their cirques (hanging, cliff, or corrie glaciers) to the great 

 Seward Glacier of Alaska, more than 50 miles long and 3 miles wide 

 where narrowest. In the Alps there are 2000 glaciers, the largest of 

 which, the Aletsch Glacier (p. 187), is more than 10 miles long, 

 although the majority are less than a mile. These Alpine glaciers 

 vary in width from a few hundred feet to about one mile. " The 

 thickness of ice in the Alpine glaciers must often be as much as 800 

 to 1200 feet," the depth usually being least at the lower end. Great 

 glaciers are confined to polar regions (continental glaciers, p. 168) 

 and to high mountains of the temperate zones. 



Position of the Snow Line. — The level on the earth's surface 

 above which some of the snow of one winter lasts through the fol- 

 lowing summer, thus forming areas of " perpetual snow " or snow 

 fields, is called the snow line. Its position depends upon the normal 

 temperature of the region as well as upon other factors. In general, 

 the snow line varies little from the line on which the average tempera- 

 ture is 3 2° F. Near the equator it is 15,000 to 19,000 feet above the 

 sea, while in polar regions it is almost, or quite, at sea level. In 



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