KANSAN DRIFT ON SUB-APTONIAX 139 



ian drift in Iowa and Nebraska, it appears that in a broad way 

 the sub-Aftonian and Kansan Lee-sheets covered approximately 

 the same territory from east to west, a territory much more ex- 

 tensive than was covered by any of the later ice-sheets in the 

 Keewatin field. 



2. Where the sub-Aftonian drift is definitely differentiated 

 from the Kansan drift, the sub-Aftonian shows a leached zone 

 considerably deeper than that of the Wisconsin drift. On this 

 1 asis the length of the Aftonian interval was considerably greater 

 than post-Wisconsin time. This is in harmony with the evi- 

 dence of the Aftonian mammalian fossils that the interval was 

 long and warm, and when both evidences are considered it is to 

 be inferred that the ice-sheet was melted back- at least to its 

 present limits. 



3. The existence of the two oldest drifts in this locality and 

 the absence of any evidence that the Maquoketa and Wapsipini- 

 con river valleys below Monticello and Anamosa, have been oc- 

 cupied by an ice-sheet, indicate that these superimposed valleys 

 have been carved since the Kansan ice invaded this region. 



4. The Kansan drift w<as weathered and eroded to its pres- 

 ent state before the loess in this locality was deposited, hence 

 this weathering records an interval much longer than post- Wis- 

 consin time. The weathering and erosion of the Kansan drift 

 does not, therefore, represent its age. The loess itself shows 

 three to four times the leaching that the Wisconsin drift shows, 

 and the length of time represented must be added to that shown 

 by the weathering and erosion of the Kansan drift, in order to 

 folly appreciate the great age of the Kansan drift. 



Department op Geology, 

 University op Washington, 

 Seattle, Washington. 



