48 



SOIL 8: PROPERTIES AND MANAGEMENT 



ice front is at a standstill; but if the wasting is rapid, 

 and the advance of the glacier is not fast enough to re- 

 place this waste, the ice is said to be retreating. A great 

 ice sheet may exhibit all three of these conditions many 

 times in its history. 





FIG. 7. Map of North America, showing the area covered by the 

 great ice sheets, the three centers of accumulation, and the approxi- 

 mate southward extension. 



35. The American ice sheet. The northern part of 

 the American continent was at one time covered by a 



