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HISTORICAL GEOLOGY 





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been obtained largely from marine, lake, and blown-sand deposits. 

 Deposits of these kinds also contain Pleistocene fossils ; but, since 

 Pleistocene animals were in existence but a comparatively short time 

 ago, their remains are also found in superficial deposits, such as river 

 terraces, peat bogs, frozen soils, ice cliffs, and cave deposits (Fig. 

 575), which, being easily destroyed by erosion, are seldom found in 

 the older formations. 



Marine Pleistocene deposits are not common, since the subsidence 

 which followed the emergent condition of that epoch buried most of 



the sediments of the 

 time beneath the sea. 

 This is unfortunate, 

 since, if a complete 

 marine record were 

 extant, we should 

 have, as the ice 

 sheet advanced and 

 retreated, a succes- 

 sion of faunas and 

 floras; the temperate 

 life changing to arctic 

 as the ice sheet ad- 

 vanced, and this, in turn, being replaced by the temperate, or even 

 subtropical life (if the climate changed to that extent), when the ice 

 again retreated. During each advance and retreat of the ice sheet a 

 migration of the life to and fro would, theoretically, be recorded. 

 Unfortunately, however, no such series of deposits has been dis- 

 covered, either because ideal conditions did not exist in any one 

 place, or because the deposits are inaccessible. 



Interglacial Deposits. — Fortunately two deposits are known which 

 were laid down during interglacial periods. In one of these, near 

 Toronto, Canada, the fossil beds were deposited upon the eroded sur- 

 face of bowlder clay and were, in turn, eroded to some extent before 

 the re-advance of the ice sheet which covered them with a layer of 

 drift. The lower portion (Don) of this deposit yields plants which 

 show that the climate was warmer in this interglacial stage than at 

 present on Lake Ontario, being similar to that of Virginia to-day. 

 This is indicated by the presence of the Judas tree (Cercis), the Osage 

 orange (Madura) and the papaw (Asimina). Besides these more 

 typically southern trees, there are maples, spruces, oaks, elms, and 



Fig. 575. — Gailenreuth Cavern, Germany. 



