580 SAMUEL CALVIN 



till, there is a petrological as well as a strat igraphical basis for separating the 



two formations 



The most notable feature of this drift sheet is its connection with the main 



deposits of loess The East-Iowan till sheet is, however, associated 



with loess of such exceptional extent and nature as to make this epoch especially 

 notable on account of this relationship. As already stated, the till graduates 

 at its edge into loess that spreads away from its border. 



The statements quoted can be interpreted in but one way. 

 They are true in case the term East-Iowan was applied to the 

 uppermost of the three till sheets in Iowa east of the Wisconsin 

 moraine. There is just one till in which "granitic types predomi- 

 nate." There is just one of which it can be said that "immense 

 bowlders are freely scattered over a portion of the surface." There 

 is just one that bears the described relation to the loess. While 

 greenstones occur in all three of the till sheets of the area under 

 consideration, it is in the middle sheet that they are most con- 

 spicuously prevalent. 



The statement on p. 756 of The Great Ice Age, clearly implied if 

 indirectly made, that "the Kansan formation emerges from beneath 

 the overlapping East-Iowan formation to the extent of 200 miles 

 at the west" can apply only to a sub-Iowan, but super- Aftonian 

 drift. It cannot possibly apply to the sub-Aftonian for the reason 

 that at the time it was written the known natural exposures of 

 the sub-Aftonian were confined to a very limited area. A number 

 of outcrops of this formation have since been recognized and 

 recorded, but it may be questioned whether the aggregate of all 

 the now known exposures of sub-Aftonian would be equal to more 

 than one or two square miles. Certainly there are no known areas 

 of anything approaching 200 miles in extent in which the lowest 

 of our drift sheets emerges from beneath anything so as to justify 

 its representation on a map. Other statements that can apply only 

 to what geologists have recently and consistently been calling Kan- 

 san occur on p. 757. The Kansan, we are told, "is greatly worn 

 in regions where the denuding agents have worked under favorable 



conditions In other regions of flat surface and low declivity 



the degradation is less marked, and extensive remnants of the 

 original surface-plane have been preserved." The first super- 

 Aftonian drift fulfils the conditions of the parts of the text quoted 

 and of many others; the sub-Aftonian does not. 



