450 HISTORICAL GEOLOGY 



Mississippi Basin evidence of several advances and retreats 

 has been discovered. The greatest extension was reached 

 by the second ice sheet, which spread southward almost to 

 the mouth of the Ohio River ; but later ones fell only a little 

 short of that. Between the several advances, the ice sheets 

 seem to have entirely disappeared or to have been reduced to 

 much smaller size. That these disappearances were caused 

 by periods of warmer climate is shown by the finding of leaves 

 of southern plants in clay beds between two layers of glacial 

 till as far north as Toronto in Canada. Trees now charac- 

 teristic of the Ohio Valley then lived abundantly north of 

 Lake Erie. 



In view of the fact that ice sheets grind down the surface 

 over which they slowly creep, we need not wonder that the 

 later ice sheets removed much of the deposits left during pre- 

 ceding glaciations. Even where the earlier sheets of drift were 

 not destroyed, they are now largely buried by deposits of the 

 later ice sheets. We therefore know the older drift best around 

 the edges of the newer. Its greater age is indicated clearly 

 by the fact that it has been deeply trenched by branching 

 systems of valleys which have been growing and extending 

 themselves through all the tune since the early deposits of 

 drift were laid down. The last drift sheet was made so re- 

 cently that the streams have barely begun this work of trench- 

 ing, and its usually rough surface is still dotted with undrained 

 lakes and marshes. 



Estimates of the length of the Quaternary period. Many 

 attempts have been made to estimate the number of years 

 represented by the glacial advances and retreats. At present 

 the cliff at Niagara Falls is being cut back several feet per 

 year. It has been calculated that at some such rate it 

 would take from 7,000 to 50,000 years to cut the entire gorge 

 below the falls. Since the falls could not have begun until 

 after the last ice sheet had retreated to Lake Ontario, a some- 

 what longer time would be required to take us back to the 

 beginning of the retreat of the latest glaciers. By compar- 



