TITLES AND ABSTRACTS OF PAPERS 77 



PLEISTOCENE DEPOSITS BETWEEN MANILLA, IN CRAWFORD COUNTY, AND 

 COON RAPIDS, IN CARROLL COUNTY, IOWA 



BY GEORGE F. KAY 



(Ahstract) 



Many deep cuts were made recently in connection with the improvement of 

 the Chicago, Milwaukee and Saint Paul Railway between Manilla, in Craw- 

 ford County, and Coon Rapids, in Carroll Countj'^ — -a distance of more than 

 thirty miles. These cuts, some of which have a depth of more than 50 feet, 

 furnish most interesting exposures of drift and related deposits, the study of 

 which has enabled some phases of the Pleistocene history of Iowa to be inter- 

 preted somewhat more clearly than was possible previously. 



The most significant features that have been revealed may be summai-ized 

 as follows : 



1. The chief kinds of material exposed are loess, Kansan gumbotil, Kansan 

 drift, Nebraskan gumbotil, and Nebraskan drift. In no one cut is it possible 

 to see all of these materials, nor are the two gumbotils exposed in a single cut. 

 In some cuts the section shows loess, Kansan gumbotil, and Kansan drift; in 

 other cuts there may be seen loess, Kansan drift, and Nebraskan gumbotil ; in 

 still others loess, Nebraskan gumbotil, and Nebraskan drift. The most com- 

 prehensive cut is about one and one-half miles west of Manning. It shows 

 loess, Kansan drift, Nebraskan gumbotil, and Nebraskan drift. 



2. The two drifts, the Nebraskan and the Kansan, are much alike litholog- 

 ically, and both appear to have undergone similar changes. On each of the 

 drifts gumbotil has been developed, below which there is a narrow zone of 

 leached drift, which grades downward into unleached drift with many concre- 

 tions. 



3. The maximum thickness of Nebraskan gumbotil is about 13 feet and of 

 the Kansan gumbotil more than 20 feet. The zone of oxidation of the Ne- 

 braskan drift is not fully exposed in any of the cuts ; the greatest depth of 

 oxidation seen was 17 feet. The zone of oxidation of the Kansan drift has a 

 maximum thickness of about 40 feet. Beneath this oxidized zone, in a few 

 cuts, there was seen less than 10 feet of very dark, tenacious, unleached, and 

 unoxidized Kansan drift. • 



4. The Kansan gumbotil is limited in distribution to a few narrow divides 

 which are erosion remnants of a former extensive Kansan gumbotil plain. 

 These divides are the present uplands of the region. The Nebraskan gumbotil 

 is exposed only in those cuts the summits of which have been brought by ero- 

 sion considerably below the elevations of the summits of the upland cuts. 



5. The loess is present as a mantle over the maturely dissected surfaces. 

 It varies in thickness from a few feet to more than 25 feet. ^ In general, it 

 thickens from the crests of the ridges down the slopes, and is apparently 

 thicker on east slopes than on west slopes. The upper parts of the ridges have 

 been broadened more than heightened by the deposition of the loess. In places 

 the loess lies on Kansan gumbotil; in places it is on Kansan drift: in other 

 places it mantles the Nebraskan gumbotil : and where there has been the most 

 extensive erosion previous to the dei)osition of the loess, it is on Nebraskan 

 drift. 



