48 



SOILS: 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. 



•0 <o 



:^ s Ms, 







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«i 



o 



n 



( ( ' r' 



(ttf£WJT/fi 

 ICE 



{Jh/VAobb? 

 < 1 1 /C£ '/> 



n 





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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 



