Fig. 470. — Map showing the glaciated area of North America. The heavy line across 

 the United States represents the limit of glaciation. The lobate outline of the 

 ice of some of the later stages is also shown. The arrows back from the edges 

 of the drift-sheets indicate direction of ice movement, as recorded by striae. The 

 clotted lines represent direction of movement generalized from the recorded striae. 

 They radiate from the three centers indicated in the text. The short arrows 

 in the western part of the United States indicate the general distribution and 

 direction of movement of the ice in that part of the continent. There were doubt- 

 less glaciers in some areas where they are not represented. 



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