130 



PHYSIOGRAPHY 



Farther west it reached almost to the southern end of Illinois. Its 

 edge crossed the Mississippi near St. Louis, and followed, in a general 

 way, the course of the Missouri River, to western Montana. Most 



Fig. 136. Sketch-map showing the area in North America covered by ice at 

 the stage of maximum glaciation. (Chamberlin.) 



of the continent north of this line was covered with snow and ice, 

 but there was an area of 8,000 to 10,000 square miles, mainly in 

 southwestern Wisconsin, over which the ice did not spread. This 

 is known as the driftless area, because there is no drift in it. There 



