170 PHYSICAL GEOLOGY 



the coast broad, shallow, basinlike depressions appear which, when 

 followed seaward, grade into great glaciers which flow from the central 

 mass into the sea through valleys. The scenery of these broad de- 

 pressions resembles, on a grand scale, the gathering grounds of 

 Alpine glaciers. The ice of the depressions which give rise to these 

 separate tongues of ice is probably a mile in thickness, 1 but on the 

 mountain ridges it is much thinner. 



The smooth almost flat central portion gives place near the 

 margins to a surface of a decidedly different character. Here, where 

 the motion of the ice is more rapid, being greater in one portion than 

 in another, the surface is much broken by crevasses which make 

 travel well-nigh impossible. Near the coast, where the mountains 

 protrude through the ice as islands (nunataks), the whiteness of the 

 surface is broken by patches and lines of rock waste derived from these 

 projections. Nunataks are often surrounded by deep ditches, due 

 to the absorption of the sun's heat by the dark rock walls and its 

 radiation from them, with the consequent melting of the adjacent 

 ice. 



As has been said, the great interior plateau is drained by glaciers 

 which descend through valleys. Many of these reach the sea, where 

 their fronts are broken off and carried away as icebergs. Some of 

 these glaciers are among the largest known. One of the most remark- 

 able is the Humboldt Glacier, which has a breadth of more than 50 

 miles where it enters the ocean. Its front rises precipitously from 

 the level of the water to a height of 300 feet, and the total thickness 

 above and below the water level at this point is probably 2700 feet. 

 Some of these glaciers fail to reach the sea but spread out on flat 

 plains. In such glaciers it is seen that the ice is stratified and that 

 the white upper layers are in marked contrast to those near the base, 

 which are often so filled with debris that it is difficult to tell where the 

 ice ends and the ground moraine begins. This loading of the basal 

 portions of the ice and the almost total freedom of the surface from 

 debris should be borne in mind when the work of the ancient ice 

 sheets is considered. 



The rate of movement of the Greenland glaciers near their ends is 

 sometimes more than 50 feet a day, but in the interior of the ice sheet 

 the rate may be as slow as an inch a day or practically zero; in other 

 words the motion is from the center of the ice sheet outward, the 

 movement being caused by the weight of the ice. Moreover, the 

 1 Geikie A., — Textbook of Geology. 



