224 THE ICE AGE IN NORTH AMERICA. 



off by the melting waters. But of this we will speak more 

 particularly when treating of the loess. 



Still outside of the Iowan and the Illinoisan deposits there 

 is a vast area in Southern Iowa, Northern Missouri, and 

 Eastern Nebraska and Kansas, which is covered with a still 

 older till, denominated Kansan. This till is much more 

 thoroughly oxidized than the other, and is spread more evenly 

 over the surface with an entire absence of moraines. Still 

 farther east (in Illinois, Indiana, Ohio, Pennsylvania, and New 

 Jersey) there is also frequently found an attenuated border 

 of glacial material which is correlated with the Kansan, from 

 its having many of the same characteristics. It is more 

 completely oxidized, more evenly spread over the area, and 

 is devoid of moraines. It now seems clear that this Kansan 

 till is the result of a movement of ice which preceded those 

 which deposited the other sheets of till and, moving east- 

 ward covered a large part of the field which is now enveloped 

 with Iowan, Illinoisan and Wisconsin deposits. The extent 

 of this eastward movement is not generally appreciated. But 

 Professor E. H. Williams found Lake Superior copper firmly 

 imbedded in " Kansan till," forty feet below the surface, 

 at East Warren, Pennsylvania, several hundred miles east of 

 the source of supply. 



A still earlier glacial deposit has been recognized by the 

 geologists of Iowa at Afton near the southern line of the 

 State, and hence called the Aftonian Episode. This is recog- 

 nized both by its position underneath the Kansan till and 

 by its excessive oxidization. But, owing to its position it 

 does not offer itself to inspection in many places, and even 

 where it is visible its true age is a matter of speculation. 



The earliest of all drift sheets is thought to have been 

 recognized by Dr. George M. Dawson in the Province of 

 Alberta, east of the Rocky Mountains, in Southern Canada, 

 while Professor Calvin has discovered what he thinks is a 

 Sub-Aftonian till in Iowa, which may correspond to that 

 described by Dr. Dawson. 



