IOWA ACADEMY 01' SCIENCES. 127 



ments of attrition of the most effective kind. The mate- 

 rials on the tops of the hills, even thongh frozen solid, could 

 not but yield rapidly to this grinding action of the ice. 

 The debris worn off from the higher points would be 

 pushed over into the valleys to the leeward of the advanc- 

 ing ice sheet. In this manner the surface inequalities 

 would gradually be reduced, both by the constant wearing 

 down of the greater elevations and by the no less constant 

 filling of the valleys witli the materials removed from the 

 tops of the hills. Owing to the short period during which 

 the flow of ice continued over this surface, the tops of the 

 Kansan hills were not subjected to the powerful abrading 

 action of great masses of moving ice for so long a period 

 as were those where the flow continued for the whole time 

 during which the lowan ice prevailed. As a consequence 

 the pre-Iowan surface here was not planed down to the 

 same extent as it was over the area covered by the main 

 sheet of the lowan drift. 



The generally smooth character of the lowan drift sur- 

 face is probably due more to the leveling action of thick 

 masses of ice moving over the region than to the amount 

 of materials transported from great distances which the ice 

 left as it retreated. The lowan ice did not generally carry 

 such a large amount of drift and debris as the Kansan, as 

 is witnessed by the comparatively thin sheet of materials 

 which is usually found covering the Kansan drift over the 

 main low^an plain. However, it is probable that the small 

 quantity of ice which melted over this lobe would be one 

 good and sufficient reason for the unusually thin mantle of 

 lowan materials that is found over its surface. 



The cause or causes which resulted in the early cessa- 

 tion of the flow of ice over this lobe did not produce their 

 full effect at once. The movement probably ceased quite 

 suddenly over the southern half of the area, but its with- 

 drawal from the northern portion was accomplished much 

 more slowly and at a much less uniform rate. A halting 

 in the retreat of the ice near the central portion of the 

 lobe, its line of lower limit receding but very slowly 

 through a long period of time, would result in the accumu- 



