334 HISTORICAL GEOLOGY 



In New York and New England no very positive evidence has 

 as yet been found to prove truly multiple glaciation, though some 

 phenomena as, for example, certain buried gorges, are difficult to 

 account for except on the basis of more than one advance and 

 retreat of the ice. At any rate there appears to be no good reason 

 whatever to believe that there were more than two advances and 

 retreats of the ice over this region. 



For our purpose in considering only the general effects of 

 glaciation, we may practically disregard the problem of multiple 

 glaciation, because the final effects would have been essentially 

 the same as a result of a single great glacial advance and retreat. 



The Driftless Areas 



In southwestern Wisconsin, and extending a little into Iowa 

 and Illinois, there is a non-glaciated area of about 10,000 square 

 miles which lies several hundred miles north of the southern limit 

 of the ice sheets (see Fig. 209). This is called a "driftless area," 

 because of the utter absence of glacial debris or any other evi- 

 dence of glaciation within its boundary. In spite of several ice 

 invasions on all sides, this small area was never ice covered. 

 Residual soils and rotten rock are widespread; there are no lakes; 

 and the streams are mostly graded and without waterfalls or 

 rapids. This small region, therefore, gives an excellent idea of 

 the kind of topography which the whole upper Mississippi Valley 

 would have shown had it not been for the glaciation. At no 

 time did the Labradorean ice sheet spread far enough eastward, 

 or the Kewatin sheet far enough westward, to cover this driftless 

 area. The highland district just south of Lake Superior doubt- 

 less served to deflect and weaken the flow of the Labradorean ice 

 which otherwise might have spread far enough to have covered 

 the driftless area. 



A much smaller driftless area has more recently been discovered 

 along the Mississippi River in Missouri. It is not difficult to 

 understand why such an area so close to the southern limit of 

 glaciation escaped all advances of the ice sheets. 



Ice Erosion 



Ice, like flowing water, has very little erosive effect unless it is 

 properly supplied with tools. When flowing ice is shod with hard 



