46 IOWA ACADEMY OF SCIENCES. 



DEGRADATION OF LOESS. 



J. E. TODD, VERMILLION, S. D. 



One of the most difficult problems connected with the loess 

 is to explain its blanket-like distribution, by which it appears 

 to be continuous over high and low altitudes alike. 



In southwestern Iowa and eastern Nebraska the altitude of 

 its base or junction with the drift varies from 100 to 200 feet. 

 As a rule its upper and lower surfaces are approximately par- 

 allel, the lower being less convex and frequently showing a 

 culmination somewhat one side of that of the upper. And 

 even occasionally there is a concavity in the lower correspond- 

 ing to a convexity in the upper. There is sometimes trace of a 

 washing of the surface of the underlying till, especially at 

 medium levels, as shown by a line of gravel or sand. The posi- 

 tion and character of the junction of the loess and drift at 

 ilower levels is not so often shown and is therefore little known. 



The generally received opinion, I think, is that the drift 

 was deeply eroded before the deposition of the loess. This 

 view as we shall see only aggravates the difficulty of the prob- 

 lem. If the surface of the drift was very uneven, as at pres- 

 ent, it is difficult to see how rivers, lakes, and winds could have 

 deposited the loess as we find it. This will appear as we pro- 

 ceed to consider the solutions which have been presented and 

 in some cases urged. These we will survej" very briefly, as our 

 time is short. 



1. The Lacustrine theory was first suggested, and for a 

 long time, for perhaps fifty years, was considered fairly satis- 

 factory. It ran through various forms, from a semi-marine or 

 estuarian origin on one hand to the result of small, local, often 

 shifting lakes, on the other. The first was forbidden by the 

 utter absence of marine forms of life, and the following objec- 

 tions lie more or less forcibly against all: 



a. There is no trace of barriers sufficient to account for their 

 existence. 



