114 ILLINOIS ACADEMY OF SCIENCE 



In the places where the loess terminates abruptly near the 

 tops of the hills, at the uppermost level of the till, the banks are 

 usually precipitous, and the streams have undercut their banks 

 and thus widened their valleys since the time of major deposi- 

 tion of the loess. In this process the loess that was deposited 

 in the earlier slopes has been removed as the work of valley 

 widening by side cutting of the streams was accomplished. At 

 these places the undercutting has taken place so recently that 

 the banks are still steep and no appreciable amount of loess in 

 excess of what has been eroded, has since been deposited upon 

 them. 



In some places where the loess continues down the 

 slopes for a considerable distance below the upper level of the 

 drift, the slopes are usually gentle, showing that undercutting 

 has not there been in progress for a long time. 



The presence of the loess over the slopes below the upper 

 level of the drift may be accounted for in two possible ways : 

 ( 1 ) It may be assumed to have been brought down from higher 

 levels by slumping and sheet wash; and (2) It may have been 

 carried up by winds from flood plain or other exposed areas 

 and deposited on the slopes where it is now found. By the 

 first assumption practically all the loess on such slopes would be 

 of secondary origin, while by the second this loess would be 

 largely in its original position. 



The first assumption is open to the following objections: 

 1. Although landslides and slumping were doubtless im- 

 portant factors in the development of the gentle slopes, after 

 undercutting had ceased, yet these processes would not result 

 in the development over the surface of a mantle of fine homo- 

 geneous loess material, unmixed with till, after the gentle slope 

 had been developed. 2. The effective action of sheet wash on 

 slopes of rather uniform gradient tends to increase with the 

 distance down slope from the top, so that any material trans- 

 ported for a distance near the top would tend to be carried to 

 the foot and not deposited on the middle or lower part of the 

 slope. 3. The thickness of the loess at the top of the hill, in 

 places where the loess continues down the slope to the flood 

 plain, is as great as it is in the places where it does not extend 

 below the highest level of the drift. An immense quantity 

 of material would be required to cover a slope % to *4 mile 

 in length to a depth of three to six feet, yet in not one of such 

 places observed was there any diminution in the thickness or 

 amount of the loess at the tops of the hills, showing conclu- 

 sively that the loess on the slopes could not have been derived 

 from the loess at the top of the hill. 



