396 IOWA ACADEMY OF SCIENCE Vol.. XXVI, 1919 



The only other way to explain the origin of the sands and gravels 

 in the light of their being of Kansan age is as follows : 



After the outwash material had been deposited, a time elapsed 

 sufficiently long enough during which the sands and gravels were 

 leached before the readvance of the ice sheet. This latter view 

 might explain the origin of the interbedded deposits. The question, 

 however, arises at once, how much time would be required to have 

 elapsed before the entire twenty feet were leached and oxidized? 

 And is there any other similar case known to glacial geologists? 

 The so-called Aftonian sands and gravels are fresh. Do they 

 really represent interglacial times? 



In the first place, it is not certain whether the twenty-foot layer 

 of sands and gravels represents the original thickness of the de- 

 posit. Thus the leaching of these materials may not necessarily 

 record the entire length of time that elapsed between the two oscilla- 

 tions of the Kansan ice sheet, as some of the leached sands and 

 gravels may have been eroded away before and at the time of the 

 second advance of the ice sheet. It seems, however, that a consid- 

 erable time must have elapsed before the uppermost till was depos- 

 ited, if we can draw conclusions from the kame and esker deposits 

 of Wisconsin age. Inspection of such well-known deposits in Wis- 

 consin show that little if any leaching has taken place since the time 

 of their deposition. Furthermore, the gravel and sand deposits south 

 of DeWitt, Iowa, which are either of Illinoian or of lowan times are 

 still fresh. These gravels as well as those of Wisconsin age have 

 no protecting cover of drift and therefore have been exposed to the 

 oxidizing and leaching processes for a considerable length of time. 

 Are we to assign a period of at least as long a duration as all of 

 post-Wisconsin time to the interval between the two oscillations of 

 the Kansan ice sheet? Hardly so. 



VIEW 8. 



The third view regarding the sand and gravel deposits explains 

 the deposits as Nebraskan outwash materials which were exposed 

 for some time to the elements of weathering before the coming on 

 of the Kansan ice sheet. 



The following steps are represented : 



1. An advance of the Nebraskan ice sheet and the deposition of 

 the lower till. 



2. The retreat and disappearance of the Nebraskan ice sheet ac- 

 companied by a deposit of outwash material. 



