174 



PHYSICAL GEOLOGY 



northeastern Illinois the average is almost 200 feet, and in the south- 

 ern peninsula of Michigan about 300 feet. Borings near Cleveland, 

 Ohio, show that the drift extends 470 feet below lake level. 



Fig. 157. — Diagram showing a region made rougher by glaciation. 



The drift deposited under the ice was often compacted by the great 

 weight of the ice mass into a dense bowlder clay which is excavated 

 with difficulty. 



Mention should be made of an area in Wisconsin and adjacent por- 

 tions of Iowa, Illinois, and Minnesota, which was not covered by the ice 



Fig. 158. — A region made smoother by. glacial debris. The uncertainty in coal 

 mining (black bands) in such a region is shown. 



sheets, the driftless area (Fig. 159 A). In this area the rock is deeply 

 weathered, and rock pillars are not uncommon ; the drainage is per- 

 fect, the streams being without swamps, lakes, or waterfalls. It 

 differs markedly in these respects from the adjoining regions (159 B). 

 This freedom from ice was due to a combination of causes : its position 

 with reference to the centers from which the ice moved, the higher 

 ground to the north, and the presence of the deep Michigan and 

 Superior basins which diverted the flow of the ice. 



Moraines. — The drift was laid down either as terminal, ground, or 

 lateral moraines, or as stratified deposits. 



{A) Terminal moraines (see p. 159) were formed where the ice 

 front remained stationary or nearly so, for a long time, so that its 

 forward movement was almost or quite equal to the melting at its 

 margin, sometimes being slightly in excess and sometimes slightly 

 less. Under such conditions it will readily be seen that the glacial 

 debris would be left in extremely irregular deposits, unless the drift 

 had been uniformly distributed throughout the ice, which apparently 

 was seldom the case. The term " recessional moraine " is often used 

 to indicate those moraines which were formed during the various halts 

 of the ice as it retreated to the north, " outer terminal moraine " 



