272 EARTH FEATURES AND THEIR MEANING 



Greenland is held in by a wall of mountains and so prevented from 

 spreading out to its natural surface in the marginal portions. 

 Through portals of the inclosing mountain ranges the out- 

 lets it sends out tongues of ice which in many respects resemble 

 certain types of mountain glaciers. 



FIG. 300. Profile in natural proportions across the southern end of the continental 

 glacier of Greenland, constructed upon an arc of the earth's surface and based 

 upon Nansen's profile corrected by Hess. The marginal portions of the profile 

 are represented below upon a magnified scale in order to bring out the characters 

 of the marginal slopes. 



Such measurements as have been made upon the inland ice 

 of Greenland at points back from, but yet comparatively near to, 

 the outlets, show that it has here a surface rate of motion amount- 

 ing to less than an inch per day, and it is highly probable that at 

 moderate distances from the margin this amount diminishes to 

 zero. Upon the outlets, on the contrary, surface rates as high as 

 59 feet per day have been measured, and even 100 feet per day has 

 been reported. We are thus justified in saying that glacier flow 

 within the outlets is from 700 to 1000 times as great as it is upon 

 the near-by inland ice, and that the glacier is in a measure drained 

 through the portals of the inclosing ranges. Back from these 

 outlet streams of ice, or tongues, the surface of the inland ice is 

 depressed to form a dimple or " basin of exudation " as is the sur- 

 face of a reservoir above the raceway when the water is being rapidly 

 drawn away (Fig. 301). 



Fissures in the ice, the so-called crevasses, are the recognized 

 marks of ice movement, and these are always concentrated at the 

 steep slopes of the ice surface in the neighborhood of its margins. 

 Upon the Greenland ice, crevasses are restricted in their distribu- 

 tion to a zone which extends from seven to twenty-five miles 

 within the ice border. 



The marginal rock islands. From its margin the ice surface 

 rises so steeply as to be climbed only with difficulty, but this 



