THE DATE OF THE GLACIAL PERIOD. 



615 



not have washed its shore for a much longer time, and the 

 smaller lakes and kettle-holes of New England and the North- 

 west can not have existed for the indefinite periods some- 

 times said to have elapsed since the glacial era, while eternity 

 itself is scarcely long enough for the development of species 

 if the rate of change is no greater than is implied if man and 

 his companions both of the animal and vegetable kingdom 

 were substantially what they are now as long ago as the date 

 often assigned to the great Ice age. 



But while approximate limits are already set to glacial 

 chronology, the held is still open for an indefinite amount of 

 painstaking inquiry. Local observers may now profitably 

 spend as much time upon a single river-valley or in a single 

 county as has yet been spent upon the whole field between 

 Cape Cod and the Mississippi. 



Fig. 156 — Bowlder bed at Poeatello, Idaho, where the Port Neuf river debouches upon 

 the Snake river plain. These bowlders were brought down to their present position 

 by the torrential floods which followed the overflow of Lake Bonneville, described 

 on pages 609 and 704. 



