ADVANCE AND RETREAT OF GLACIERS IO9 



of snow. The snowfall is not very great ; but so little of it is 

 disposed of by evaporation or melting, that there is a large excess 

 which goes to the growth of the ice-sheet, and keeps up the supply 

 for the innumerable glaciers which flow to the sea. 



The source of a glacier is always above the snow-line, but the 

 ice-stream itself may descend far below that line, slowly melting 

 and diminishing in thickness as it flows. The lower end is at the 

 point where the rate of melting and the rate of flow balance, so 



Fig. 36. — Nunatak rising through the ice-cap, Greenland. (Photograph by 



Libbey.) 



that changes in the temperature of the seasons or in the amount 

 of the snow supply will cause the glacier to advance or retreat, as 

 one or other of these factors prevails. Thus the Alaskan glaciers 

 have retreated notably within the last century, while some of the 

 Norwegian ones are advancing. From the lower end of a glacier 

 there always issues a stream of water, which flows under the ice, 

 often in great volume, and even in winter, for the thick ice is a 

 non-conductor and protects the stream from the intense cold 

 of the air. 



