THE PLEISTOCENE OR GLACIAL PERIOD. 



385 



ties named, great masses of this assorted material were plowed up 

 by the succeeding Kansan ice-sheet and incorporated in its till, as shown 

 in Fig. 511. The organic remains in the interglacial beds seem to 

 imply a cool temperate climate, but as a cool temperate stage must 

 be passed through twice in every transition from a glacial climate 

 to a warm one and back again, organisms indicating a cool climate 



Fig. 510. — Section of drift at Thayer, Union County, la. The stratified drift below, 

 making up the larger part of the section, is Aftonian. It is overlain by Kansan 

 till. (Calvin, Iowa Geol. Surv.) 



do not necessarily show how great an amelioration may have been 

 reached, unless the record is known to be complete. The length of 

 the Aftonian interval has not been well determined, from lack of ade- 

 quate accessibility, but it was at least a notable interval. The pebbles 

 are much decayed and the soils, peat, etc., imply a considerable lapse 

 of time. 



The old drift in western Pennsylvania doubtfully referred to the 

 sub- Aftonian stage is in somewhat like manner associated with impor- 



