IOWA ACADEMY OF SCIENCES. g5 



discovered by Prof. J. A. Udden, and has been visited by Pro- 

 fessor Calvin, Dr. Bain and the writer, each of whom recognize 

 the presence of both sheets of drift, and also the Yarmouth 

 weathered zone. The surface of the Kansan till sheet has the 

 appearance of slight erosion, for it shows a rise of about fifteen 

 feet in a distance of twenty or thirty rods. The lUinoian till 

 sheet-rests uncomformably upon the eroded Kansan, reaching a 

 lower level at the south end of the exposure than at the north. 

 In making the descent along Eighth street the following series 

 of beds was found: 



FEET. 



Loess 30 



Weathered zone of reddish-brown till (Sangamon) . 3 



Unleached brown till (lUinoisan) 15 



Weathered zone of gummy, gray clay (Yarmouth). 3 

 Brown till changing to gray color at 12 to 15 feet 



(Kansan) 30 



EXPOSURES IN ADAMS COUNTY, ILLINOIS. 



The most southerly exposures of the Yarmouth weathered 

 zone yet observed are in Adams county, Illinois. In a ravine 

 in Woodville, in the northern part of the county, two sheets of 

 brown till appear, which are separated by a gray, gummy clay. 

 This clay is thoroughly leached while the till immediately 

 above it is unleached. The latter has a thickness of only ten 

 or twelve feet. Another exposure was found at a well in pro- 

 cess of excavation on a farm eight miles east of Quincy. This 

 section is similar to that in the ravine except that the Illinoian 

 till sheet has a thickness of twenty feet. Another exposure 

 was found north of Payson near the base of an Illinoian drift. 

 The gray clay here rests upon a gravelly bed instead of a 

 sheet of till, but appears to be of similar origin and age to the 

 other beds referred to the Yarmouth stage. 



Within a few miles south from this exposure the border of 

 the Kansan drift emerges from the edge of the Illinoian, and 

 passes southward into Missouri. 



The driftless peninsula found by Professor Salisbury, here 

 sets in and occupies a narrow strip west of the Illinois, from 

 Pike county to the mouth of that stream,* beyond which the 

 margins of the Illinoian and Kansan sheets take widely diver- 

 gent courses. Fortunately there was sufficient overlap north 

 from this driftless peninsula to make clear the interj)retation 



*See Proc. A. A. A.. S., Washington meeting, 1891, pp. 251-253. 



