THE IOWAN LOESS. 157 



the plains between the streams has been known for only a few years. In 

 the region under discussion the first recognition of the interfluvial silts as 

 a deposit distinct from the glacial drift appears to have been made by 

 Professor Salisbury in his studies on the borders of the Driftless Area in 

 1885 and in southern Illinois at a later date. The discussion of loess in 

 the Illinois geological reports apply chiefly to the deposits on the immediate 

 borders of the main valleys, though some recognition of its extent into 

 the region about Springfield appears to have been made by Professor 

 Worthen and his associates. 



The structure of the loess varies in vertical sections as well as from 

 place to place. The leading variations in the vertical sections are such as 

 to support a threefold division : (1) The surface portion, 2 to 4 feet in 

 depth, which has an earthy structure due probably in part to the breaking 

 down of many of the grains under atmospheric action. This phase charac- 

 terizes not only the deposits on interfluvial tracts, but also those on the 

 borders of the main valleys, as is natural if the earthy appearance is due to 

 atmospheric action. (2) The main body of the loess, which is a silt usually 

 without definite bedding planes or stratification. It is somewhat more 

 porous on the borders of the main valleys than beneath the interfluvial 

 tracts. The variation in texture is apparently due to the removal of the 

 finer material on the borders of the valleys rather than to the presence of 

 coarser material there than in the interfluvial tracts. (3) The basal por- 

 tion, which commonly shows a more distinct bedding than the body of the 

 loess, and is in places sandy and pebbly. As a rule the pebbles are con- 

 fined to the lower 2 or 3 feet, but in the thicker portions of the loess the 

 well-defined bedding may occupy a depth of several feet. The pebbles 

 often occur in places where the bedding is not distinct, Indeed, the most 

 distinctly bedded portions are usually almost free from pebbles. 



In following the loess from place to place across the interfluvial tracts 

 it is found to undergo gradual changes in texture and color, for which a 

 cause is not in all cases manifest. But as a rule the more porous portions 

 of the loess are found in proximity to a large valley or to the border of the 

 Iowan drift sheet. Upon passing back from the valleys the open texture 

 becomes less pronounced, and there is a gradual change to a clayey loam 

 and then to loamy clay. A similar change in texture is found in passing 

 away from the border of the Iowan drift sheet. This is well shown both in 



