EFFECT OF THE GLACIERS 



19 



Effect of the Glaciers. These invading and melting ice sheets 

 produced profound effects upon the country over which they 

 moved. In thickness they varied from a few feet at their margins 



FIG. 9. Map showing the extent of the ice sheet over North America. Note, the 

 driftless area, covering a portion of what is now Wisconsin, Iowa and Illinois. (Chamber- 

 lain and Salisbury.) 



to several thousand feet at their centers. They were thick enough 

 to slide over the high mountains of the New England States. 

 In their movement rocks were ground to powder, the pre-exist- 

 ing soil mantle was swept away, some old valleys were filled 

 up and new ones were gouged out, rivers were turned from their 



