94 



GEORGE F. KAY AND J. NEWTON PEARCE 



thought them to be related to loess. Until recently McGee was 

 the only geologist who had stated definitely that the material 

 which is now called gumbotil is the product of weathering of 

 drift. 



FIELD STUDIES OF GUMBOTILS AND RELATED MATERIALS 



The conclusion which was presented by Kay in his paper in 

 Science is : 



.... that the gumbotil is the resiilt chiefly of the chemical weathering 

 of drift was reached only after the field relations of gumbotil had been studied 

 carefully, and detailed chemical analyses of Nebraskan gumbotil, Kansan 

 gumbotil, lUinoian gumbotil, and the glacial tills underlying these gumbotils 

 had been made. 



The field relations of Kansan gumbotil to the underlying 

 Kansan till have been already briefly described.^ The Kansan 

 gumbotil, there called super-Kansan gumbo, reaches a maximum 

 thickness of more than twenty feet, and is limited to tabular 

 divides and other remnants of a gumbotil plain which, before it 

 was affected by erosion, was as extensive, apparently, as the original 

 Kansan drift plain. This gumbotil occupies a definite topographic 

 position, and where it is exposed in railroad cuts it is seen to lie 

 horizontally in the cut and not to conform to the surface slopes 

 which have been developed by erosion. The gumbotil is dense, 

 sticky, and very slippery when wet, but is hard and very tenacious 

 when dry. It is usually dull gray to drab in color; in places the 

 gray color is mottled with brown and reddish tints. It is leached, 

 but in many places it contains lime concretions. The dry surfaces 

 of the exposures of gumbotil are distinctly checked by sun cracks. 

 It contains only a few small, scattered pebbles, which consist 

 predominantly of quartz and chert and subordinately of crystallines 

 and quartzites. A striking feature of the quartz and chert pebbles 

 is their remarkably smooth surfaces. The gumbotil grades down- 

 ward into yellowish to chocolate-colored till, in many places with 

 numerous pebbles, few if any of which are calcareous. This 

 oxidized and non-calcareous till, in turn, merges into unleached 



' George F. Kay, "Some Features of the Kansan Drift in Southern Iowa," Bull., 

 GeoZ. ^oc. ^mer., Vol. XXVII, pp. 115-17; reprinted in /owo Get?/. Surv., Vol. XXV, 

 pp. 612-15. 



