48 



SOILS- PIIOFEUTIES AND MANAGEMENT 



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

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

 place thib \^aste, the ice is said to be retreating. A great 

 ice sheet may exhibit all three of these conditions many 

 times hi its history. 



Fia. 7 — Map of Noith 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 



