Geography of Ohio 207 



sheet; thus we infer that animals had reoccupied the area after 

 the former ice had gone. 



Each extension of ice did not attain the identical marginal limits 

 of the preceding sheet. For example, the glacier of the last epoch, 

 the Wisconsin, throughout part of the glaciated area, extended 

 beyond the limits of any preceding ice sheet. This, however, 

 was not the universal condition. The imbricated relationship 

 exists at sufficiently scattered points to indicate very satisfac- 

 torily the conclusions about the succession of the drift sheets. 



While we more commonly assert that the early sheets of drift 

 are more weathered than are the later, at the same time, it must 

 be borne in mind that the better understood evidences of weather- 

 ing may be more obvious in the deposits of later ice sheets. Oxi- 

 dation, particularly of deposits with a ferruginous content, gives 

 rise to the rusty and yellow appearances usuall}' asociated with 

 weathering. The lower parts of an earlier drift sheet, which had 

 not been reached by weathering agents before the area was spread 

 over by a later ice sheet and permanently buried, may show much 

 less indications of weathering than does the surface part of the 

 most recent drift sheet. Furthermore, the early over-ridden 

 deposits would be made compact, and percolating ground water 

 might more closely cement its parts, thus indurating it. Again, 

 this early drift, w^hen over-ridden by later ice, would suffer strains 

 and stresses, and joints and faults would be induced thereby. 



Where a post-glacial river has cut a channel through succeeding 

 drift sheets, or where the wave work of lakes has developed a cliff 

 exposing different drift sheets, we have the best opportunity of 

 studying their appearance. When a later sheet failed to reach the 

 margin of an earlier sheet, we can best observe the different 

 extent of stream development on their surfaces. 



OTHER GLACIAL PERIODS 



It is not many years since geologists gave general credence 

 to the scattered reports of workers in South Africa, India and 

 Australia, concerning the existence of glacial conglomerates 

 belonging to the Permian and Cambrian periods. Many had 

 supposed that the Pleistocene glacial epoch was itself convincing 

 proof of the earlier theory of molten interior and the cooling crust 



