138 IOWA ACADEMY OF SCIENCE 



5. Till, brownish yellow to yellow, leached 6 feet, cal- 

 careous and limestone pebbles below, upper 

 horizon conforms to the contour of the hill and 



mantled by the loess, maximum thickness 20 



Grades downward into: 



4. Till, gray-blue, calcareous, occurs below thickest 

 part of the oxidized till and in a small de- 

 pression of the underlying gravel; maximum 

 thickness 8 



3. Sand and gravel body, extends across the cut except 

 where mantled on the slopes by loess; limestone 

 and other drift pebbles present, yellowish to 

 brownish in color, lens and pocket-structure; 

 thickness 10-12 



2. Till, light drab to dark drab at the top, brownish 

 gray to dark gray below, dense and compact, 

 limestone pebbles to the top, mostly decayed in 

 the upper one foot, contains some inclusions of 

 the underlying silt; thickness 12-14 



1. Silt, somewhat sandy, yet compact, dark gray, no 

 pebbles, fragments of wood or roots of wood 1 to 

 2 inches in diameter exposed near the bottom, 

 upper horizon somewhat undulating and in places 

 shows gouging by an over-riding ice-sheet; 

 thickness 12± 



Interpretation: — There is no zone of leaching and oxidation 

 within the drift materials which warrants a separation into 

 two distinct tills. It has been thought, however, in view of 

 the other exposures, that possibly the gravel-body represents 

 such an interval as the Aftonian and that the overlying drift is 

 Kansan in age and the underlying is sub-Aftonian. In this 

 case, the bottom silt formation would be probably pre-Pleisto- 

 cene. These determinations, however, must remain somewhat 

 conjectural. 



It is quite clear, however, that the loess formation was depos- 

 ited on the Kansan drift after the latter had been eroded and 

 weathered to its present state. This means a relatively long 

 interval between the deposition of the two as compared w r ith post- 

 Wisconsin time. 



SUMMARY OF THE CHIEF POINTS. 



1. These exposures definitely record the invasion of the sub- 

 Aftonian ice-sheet into the extreme eastern part of Iowa. Tak- 

 ing into consideration the other known exposures of sub-Afton- 



