THE CONTINENTAL GLACIERS OF POLAR REGIONS 273 



gradient steadily diminishes until at a distance of between seventy- 

 five and a hundred miles its slope is less than two degrees. Where 

 crossed by Nansen near latitude 64 N. the broad central area of 

 ice was so nearly level as to 

 appear to be a plain. 



As we pass across the irregu- 

 lar ice margin in the direction 

 of the interior, larger and larger 

 proportions of the land's sur- 

 face are submerged, until only 

 projecting peaks rise above 

 the ice as islands which are 

 described as nunataks (Fig. 

 302). 



Though not a universal ob- 

 servation, it has been often 

 noted that the absorption of 

 the sun's rays by rock masses 

 projecting through the snow 

 results in a radiation of the 

 heat and a lowering by melting 

 of the surrounding snow and 

 ice. For this reason nunataks 

 are often surrounded by a deep 

 trench due to a melting of the 

 snow. Such a depression in 

 the ice surface about the mar- 

 gin of a nunatak, from its re- 

 semblance to a trench about 

 an ancient castle, has been 

 designated a moat (Fig. 303). 

 For the same reason, the out- 

 let tongues of ice which descend 

 in deep fjords between walls of 

 rock are melted away from 

 the walls and a lateral stream 

 of water is sometimes found 

 to flow between ice and rock 



on ice - 



On land 



Kilometer; 



(pi. 13 B). 



FIG. 301. Map of a glacier tongue, with 

 dimple showing above and due to in- 

 draught of the ice. Umanakf j ord, Green- 

 land (after von Drygalski). 



