THE ORIGIN OF GUMBOTIL 97 



In all of these sections the zone of oxidized and leached till 

 beneath the gumbotil is narrow. A study of thirty-five sections 

 widely separated as to location shows that in eighteen of them the 

 zone of oxidized and leached Kansan till beneath the Kansan 

 gumbotil is 5 feet. In twelve of them it is 5 feet 6 inches; in the 

 remaining sections the zone is somewhat more than 5 feet 6 inches 

 or sHghtly less than 5 feet. The uniform thickness of the leached 

 zone is impressive. The thickness of the oxidized, unleached 

 zone of Kansan till is about 40 feet. 



The distribution of Kansan gumbotil in Iowa. — The relations of 

 Kansan gumbotil to the underlying Kansan till have been seen at 

 scores of places in southern Iowa and at many places in other 

 parts of the state. In fact the Kansan gumbotil has been studied 

 in every county of three tiers of counties in southern Iowa as well 

 as in many of the counties which are farther north.' Moreover, 

 within the lowan-drift area the Kansan gumbotil has been found 

 beneath lowan drift at numerous places.^ It will be of interest 

 to state that the Kansan gumbotil is now known at a sufficient 

 number of places in Iowa to permit the restoration of the Kansan 

 gumbotil plain, that is, the original plain surface of the weathered 

 Kansan till, as it was in Iowa before any great erosion was accom- 

 plished. 



Some sections showing the relations of Nebraskan gumbotil to 

 underlying Nebraskan drift. — -The field relations of the Nebraskan 

 gumbotil to the underlying Nebraskan till are similar to the rela- 

 tions that have been described as existing between the Kansan 

 gumbotil and the underlying Kansan till. The two tills, the 

 Nebraskan and the Kansan, are much ahke hthologically and 

 both appear to have undergone similar changes under similar 

 conditions. Below the Nebraskan gumbotil there is, as in the 

 case of the Kansan gumbotil, a narrow zone of leached, oxidized tOl 

 which grades downward into unleached, oxidized till with many 

 concretions. 



'George F. Kay, "Pleistocene Deposits between ]\Ianilla in Crawford County 

 and Coon Rapids in Carroll County, Iowa,"' loK'a Geol. Sitrv., Vol. XXVI (1917), 

 pp. 215-31. 



=> W. C. Alden and M. M. Leighton, "The lowan Drift, a Review of the Evidences 

 of the lowan Stage of Glaciation," Iowa Geol. Surv., Vol. XXVI (1917), PP- 92-109. 



