113 ILLINOIS ACADEMY OF SCIENCE 



From the detailed study of these and other areas it seems 

 certain that the development of erosion slopes on the surface 

 of the Illinoian drift previous to the deposition of the loess 

 was several times that at present attained on the surface of 

 the Early Wisconsin drift, indicating a correspondingly longer 

 interval for its accomplishment than has elapsed since the 

 withdrawal of the Wisconsin ice sheet. 



It might be suggested that the surface of the Illinoian till, 

 beneath the loess, reproduced the topography of the pre- 

 Ulinoian surface as a result of the present streams following 

 pre-Illinoian channels that were not completely filled by the 

 Illinoian till. Some of the larger streams in the Canton quad- 

 rangle are following pre-Illinoian channels, but only in a part 

 of their courses, and many of the smaller streams have no 

 place found buried channels. However, the slopes of the 

 Illinoian till, beneath the loess, bordering the portions of the 

 stream valleys, where they do not follow pre-Illinoian chan- 

 nels, are just as gradual as in the places where they do. The 

 thickness of the Illinoian till in this region averages about 

 24 feet, which depth is sufficient to entirely obliterate all of the 

 smaller pre-Illinoian valleys and many of the larger ones also, 

 as the field study shows it has done. Hence the generally 

 gradual slopes of the surface of the till, beneath the loess, 

 bordering the smaller as well as the larger streams, could have 

 been developed for the most part only by stream erosion on the 

 surface of the Illinoian till, before the overlying loess was laid 

 down. 



TIME OF THE MAIN LOESS DEPOSITION 



In the Mississippi Valley and elsewhere dust is at present 

 being carried and deposited by the winds, and doubtless under 

 favorable conditions of gathering and lodgment, the winds 

 have carried and deposited such materials throughout past 

 geological periods. It seems certain, however, that peculiar 

 conditions unusually favorable for loess deposition, prevailed 

 over the Mississippi valley during very late Iowan and early 

 Peorian stages of the Pleistocene. This is shown, ( 1 ) by the 

 intimate relations of the loess to the border of the Iowan drift 

 plain above described, and (2) by the very slight development 

 of loess on the surface by the Iowan and Wisconsin drift sheets 

 compared with its much greater thickness over the older Kan- 

 san and Illinoian till. 



That the most important loess deposition did not take place 

 after the Iowan period of glaciation is shown by the ab- 

 sence of anything like such thickness of loess over the Iowan 



