192 ILLINOIS STATE ACADEMY OF SCIENCE 



flour have been leached from the upper several feet. That 

 peculiar and distinctive product of long-continued weath- 

 ering of till, gumbotil, is exposed in several places be- 

 neath the mantle of loess. 



The maximum thickness of the drift in this quadrangle 

 remains to be determined from a study of well records, 

 but a striking feature of the area is the abundance of rock 

 outcrops wherever slopes have been washed or gullied. 

 In many places the bed rock is only thinly veneered with 

 till, and on many ridge crests the present thickness of the 

 till is not more than a few feet. Over much of the quad- 

 rangle the original thickness of the drift was not ade- 

 quate to mask the topography of the rock surface, so that 

 pre-Illinoian valleys and divides can be traced in the 

 existing topography. 



Borings show that the flattish uplands west of Rock 

 River are underlain by loess to a depth of 15 or more 

 feet. The thickness is much less on slopes, but this un- 

 doubtedly is due in large part to wash and creep during 

 and subsequent to deposition of the loess. Traces of an 

 old soil bed and fragments of vegetation are present be- 

 tween the loess and the underlving weathered Illinoian 

 till. 



The only recognizable deposit of Early Wisconsin age 

 in this area consists of alluvium deposited in the eastern 

 part of the Oregon Basin. It was made by streams that 

 were blocked by outwash from the Early Wisconsin ice- 

 sheet when it covered the region east of this quadrangle. 

 Some of the loess may have been deposited during this 

 * epoch, but the amount is probably relatively insignificant 

 when compared with the pre-Early Wisconsin loess. 



The late Wisconsin deposits consist in the main of val- 

 ley train material that partially filled Rock Valley and 

 of alluvium that was deposited in tributaries which were 

 blocked by the drainage down Rock River. Other streams 

 have made their normal deposits up to the present time. 

 Winds have whipped up some of the sand into low dunes 

 in Wisconsin in recent times, and meager talus slopes 

 have been formed along some of the cliffs. 



Structure. — The structure of the formations in the 

 Oregon quadrangle departs markedly in many places 



