LEACHED GRAVEL DEPOSITS 397 



3. A long interglacial period during which the outwash material 

 was thoroughly oxidized and leached. 



4. An advance of the Kansan ice sheet and the deposition of the 

 upper drift. 



5. The disappearance of the Kansan ice sheet. 



Discussion. — This view is the only one which will satisfactorily 

 account for all the conditions as they exist in the field : namely, an 

 upper till the top of which is oxidized and leached and whose lower 

 portion is fresh, then an oxidized and leached stratified deposit of 

 sand and gravel beneath which is an unoxidized fresh till. Pre- 

 sumably as the Kansan ice advanced over the outwash deposits, the 

 sand and gravel were frozen into a solid mass. 



OTHER EVIDENCE BEARING ON THE PROBLEM 



A study of the gumbotils in close proximity to the described sec- 

 tions shows that there are two distinct gumbotils, one lying at an 

 elevation of about 710 feet and the other from 620 to 640 feet above 

 sea level. The higher lying gumbotil is the Kansan, and the lower 

 one the Nebraskan. There never has been any doubt regarding 

 the Kansan gumbotil, but it has just recently been established that 

 the lower gumbotil is the Nebraskan. Now the noteworthy point is 

 that the elevations of the sand and gravel deposits are approximately 

 the same as those of the Nebraskan gumbotil near by. This coin- 

 cidence of elevations seems to signify that the sands and gravels 

 were lying on the same surface as that of the Nebraskan drift and 

 that while the till was being oxidized and leached to form the Ne- 

 braskan gumbotil, the sands and gravels were also undergoing the 

 same processes of leaching arjd oxidation and hence represent the 

 same interglacial interval. 



CONCLUSIONS 



We have here the following facts : 



1. The stratified sands and gravels are oxidized and leached. 



2. These deposits lie between two fresh drifts of which the upper 



part of the overlying till is leached, hence presenting a section 



as follows: 



Drift leached 



Drift unleached 



Sand and gravel deposit leached 



Drift unleached 



3. There are two such exposures, separated by several miles and 

 having the same elevation as well as the same stratigraphic 

 and topographic relationships. 



