100 IOWA ACADEMY OF SCIENCES. 



notably earlier than the overlying drift. At Afton they seem 

 to represent kame-like aggregations, but whether made during 

 the advance of the Kansan, or the retreat of the pre-Kansan, is 

 not entirely certain. In general the waterlaid beds are such as 

 might have been formed by agencies closely connected with 

 the ice. The possible exception is the buried loess at Afton 

 Junction, which, however, would only necessitate a considera- 

 ble change in the vigor of deposition between the time of its 

 formation and the laying down of the overlying gravel. 



Fifth. — Since the presumed sub-Aftonian drift is thought to 

 be wholly covered by the Kansan, and is certainly known to be 

 in the region studied, there is but little chance to contrast the 

 topographic development of the two drift surfaces. Relative 

 to erosion in the period between the two drift sheets it may be 

 stated that the Hastie exposure strongly favors such a suppo- 

 sition. The evidence pro and con at Afton exposures is in too 

 uncertain a condition to warrant any conclusions. It may be 

 said, however, that there is much which indicates a notable 

 period of erosion and very little, if any, evidence against it. 



Sixth. — It has been shown that there are exposures in the 

 region of a drift of peculiar physical type That this drift is 

 wholly unlike any known phase of the Kansan, and that in 

 every instance there are some independent phenomena favoring 

 the hypothesis that it is distinctly older than the Kansan. 

 Whatever one may think of correlations based upon physical 

 characters these facts are certainly of some significance. Fur- 

 thermore the same facts are true of the known exposures of 

 the presumed pre-Kansan drift at Muscatine, Oelwein, Albion, 

 and indeed throughout the state. 



General Conclusion.- — It is believed that the argument for a 

 pre-Kansan drift sheet derived from erosion is strong, and that 

 it has independent value. The arguments from other sources 

 tend to greatly strengthen it, and the cumulative force of the 

 whole is believed to be sufficient to put the burden of proof 

 upon those, if any, who would attempt to deny the existence of 

 a pre-Kansan drift. All would, however, probably agree to 

 the statement which the writer believes warranted by the evi- 

 dence in hand, and which he expects future investigations to 

 amply confirm, but for anything beyond which there is prob- 

 ably as yet no sufficient evidence: that there are in Iowa traces 

 of a drift sheet older than the Kansan and separated from it by 

 an unknown, but probably considerable, interval. 



