544 IOWA ACADEMY OF SCIENCE Vol. XXV, 1918 



consin ice ploughed it up and mixed it so thoroughly with the 

 Wisconsin drift as to destroy all its identity; (2) the lowan 

 ice sheet may have extended no further west than the east mar- 

 gin of the Wisconsin drift; (8) the time interval between the 

 retreat of the lowan ice sheet and the advance of the Wisconsin 

 ice sheet may have been of sufficient length to permit of the 

 removal by erosion of the lowan drift before the advance of 

 the Wisconsin ice, or (4) the lowan drift does exist beneath 

 the Wisconsin, but as yet has not been disovered. 



Department op Natural Science, 

 State Teachers Collecje. 



