220 abstracts: geology 



The Pleistocene deposits consist of remnants of a probably early drift 

 of unknown age, lUinoian drift, Wisconsin drift, loess, and postglacial 

 alluvium. At several localities dark bowlder clay was noted, which re- 

 sembles in some respects an old till occurring under the Illinoian and the 

 Kansan drift sheets in Iowa. This bowlder clay contains a smaller 

 percentage of dolomitic limestone pebbles among the smaller sizes than 

 is normally present in the Illinoian drift, and it is associated with silt 

 that commonly occurs in association with the older drift in Iowa. The 

 till itself contains large pockets of yellow sand, such as are common in 

 the old tills west of the Mississippi. The till in these places is of un- 

 kno-wTi age, but it differs considerably from the local Illinoian till in its 

 general appearance and probably represents an earlier invasion of the 

 ice. 



The till resulting from the Illinoian ice invasion was no doubt origi- 

 nally laid down over the entire area of this quadrangle. In the eastern 

 part it is overlain by drift of Wisconsin age wherever the latter was not 

 removed by interglacial erosion or worked into the later till. The thick- 

 ness of the Illinoian till varies from about 20 to nearly 80 feet. On its 

 surface is a soil formed in interglacial time and kno^vn as the Sangamon 

 soil. Outside the limits of the Wisconsin drift the weathered and eroded 

 surface of the Illinoian till is covered by loess. In places also similar 

 loess is exposed intercalated between the weathered Illinoian till and the 

 overlying Wisconsin till showing that a part at least of the super-Illinoian 

 loess is of pre- Wisconsin age. 



The Wisconsin drift is limited to the eastern and northeastern half 

 of the quadrangle. Its margin is traceable through the central part of 

 the region, though the terminal moraine is not strongly marked. 



In places remnants of two former flood plains border the present 

 bottom lands of the streams. Both of these date from late stages of the 

 Wisconsin ice invasion. The older terrace has an average elevation of 

 520 feet above sea level. The surface of this terrace has been consider- 

 ably modified by erosion. The later Wisconsin terrace has an average 

 elevation of 476 feet above sea level and forms a narrow strip bordering 

 the older terrace. 



Loess, a dusty yellow material, generally not calcareous in this area, 

 covers all of the upland, overlying both the Illinoian and Wisconsin 

 drift sheets. Its upper part is dark from decayed vegetation, forming 

 the upland soil. Directly under the soil the loess in many places is im- 

 pregnated with ferruginous material, which gives it a reddish-brown 

 color in various shades. In other places, especially where the ground 



