82 IOWA ACADEMY OF SCIENCES. 



A THEORY OP THE LOESS. 



B. SHIMEK. 



Some years ago in an article entitled "The Loess and Its 

 Fossils,"^ the writer advanced certain opinions the modilica- 

 tion of which seems to be called for by subsequent investigation 

 and thought. 



In that paper it was shown, principally from a study of the 

 fossils, that the theory of the lacustrine origin of the loess, 

 held with very few exceptions by American writers,'- is unten- 

 able, and that the origin of the loess in violent fluviatile floods, 

 also sometimes suggested, is equally improbable, and the 

 theory was there offered that the deposit was formed in ponds 

 and lakes similar to those which were formerly abundant in 

 northern Iowa, and by quiet overflows of the sluggish prairie 

 streams. 



Although it is extremely probable that certain limited por- 

 tions of the unmodified loess were deposited in this manner, 

 the theory does not account for the most extensive deposits 

 which usually cap the highest hills, especially along our streams 

 which so often seem to cut their channels through the highest 

 ridges. This difficulty led the writer to further investigation, 

 which led to the conclusion that wind was the prime agency 

 concerned in the formation of these deposits, and that Rich- 

 thofen's theory of the formation of the Chinese loess, tempered 

 and modified in important particulars, will account for all the 

 phenomena of the loess of the Mississippi valley. 



That the loess is not of aquatic origin is indicated by the 

 following facts: 



WuU. Nat. Hist. S. U. I., Vol. II, pp. 93-98. 



'^Vrof. C'Alvin, in Iowa Geol. Survey, Vol. IV, p. 81, recently suggested the aeolian 

 origin of a part of the loess in Allamakee county. 



