PAPERS ON GEOLOGY 



107 



The general distribution of the loess, being thicker and 

 coarser on the hills bordering the larger streams than over 

 interstream areas (see figure 2), and especially thick on the 

 tops of the hills, bordering the windward side of the valleys 

 and around the border of the Iowan drift plain, in Iowa, and 

 towards the north in Iowa and northern Illinois than farther 

 south; the great range of relief shown in the loess deposits, 

 being more than 600 feet in Illinois, and exceeding a thousand 

 feet in the Mississippi basin; the fact that the loess does not 

 tend to level the inequalities of the surface, but mantles the 

 hills, prairies and lowlands ; the general lack of stratification 

 or' lamination of the deposits ; and the presence in the loess 

 of entire shells (many of which are fragile and easily broken) 

 of species of terrestrial gastropods that now live on dry wood- 

 land hills, are conclusive evidences that the loess in the Missis- 

 sippi Valley region was carried and deposited by wind, rather 

 than by water. 



Water laid silts are distinguishable from true loess by the 

 greater distinctness and horizontality of their stratification, 

 by the more heterogeneous character of their constituent ma- 

 terials and by the fact that their fossils, when present, are 

 aquatic. If sediments possessing these characters of aqueous 

 deposition were never included under loess deposits, it would 

 tend to clear up and prevent much of the confusion that is now 

 so prevalent concerning the loess. There is no more reason for 

 designating water laid deposits resembling loess in texture, or 

 even derived from loess by the name loess, than there is for 

 applying the name shale to fluviated deposits derived from 

 beds of shale. In geologic mapping all such water laid de- 

 posits would be classed as alluvium. 



Calvin 8 has pointed out that there are three things to ac- 

 count for in the proper explanation of loess deposition : 



1. An extensive gathering ground of bare and dry surfaces 

 as a direct source of the material. 



2. An agent of transportation and deposition consistent 

 with the source of supply and the sites of deposition. 



'3. Obstructions to the transporting agent, such as would 

 result in deposition and lodgment in the places where such 

 deposits now occur. 



All the above conditions are completely satisfied in harmony 

 with deposition of the loess by winds. The position of the 

 thicker and coarser loess deposits on the bluffs bordering the 

 windward side of the larger flood plains are in the places where 

 the prevailing westerly winds, after sweeping over exposed 



8. Samuel Calvin: The Iowan Drift. Journal of Geology, Vol. XIX, No. 7, 

 1911, p. 601. 



