MODE OF DEPOSITION OF THE LOESS. 181 



confident that he lias seen not a few places where the basal portion of 

 nnmodified loess carries sand and coarser material. It is found that 

 on the borders of the Mississippi Valley in southeastern Iowa, and also for 

 some distance back from the stream, pebbles are of frequent occurrence in 

 the lower portion of the silts, which there cap the Sangamon soil or rest 

 upon the slightly eroded surface of the Illinoian till. Well-defined bedding 

 planes also appear, especially in the basal portion of the silt, in which thin 

 sand partings alternate with clay, giving an appearance similar to that 

 found in the fine deposits of alluvium on the bottoms bordering the large 

 streams. These pebbles and bedding planes have been found in Lee and 

 Des Moines counties, Iowa, at an elevation of slightly more than 700 feet 

 above tide, or fully 200 feet above the present level of the Mississippi River, 

 and about 130 feet above the level of the lowest known deposits of loess in 

 that region. The breadth of the glacial waters along the Mississippi Valley 

 must have been 25 to 50 miles if the deposits just noted mark the highest 

 level; possibly a still higher level was reached. Whether there was clear 

 evidence of the spreading of glacial waters over the entire divide between 

 the watersheds of the Mississippi and Illinois, can not be stated. But the 

 known extent of the glacial waters seems adequate to have furnished material 

 for mantling the higher portions of this divide by seolian agencies. Exami- 

 nations in southern Illinois have brought to light the general prevalence of 

 small pebbles in the silt which covers that district. There seems little ques- 

 tion that from the base of the elevated ridge in southern Illinois northward, 

 as far as these silts are exposed, outside the Wisconsin drift, the aqueous 

 agencies have been influential and were probably the chief agencies of 

 deposition. 



To what extent the elevated ridge in southern Illinois was covered by 

 glacial waters has not been determined. Possibly a portion of the silt which 

 caps this ridge was deposited while the Illinoian ice rested upon its northern 

 slope, though no positive evidence of such deposition has been noted. 



The extent of the glacial waters on the borders of the Mississippi in 

 northwestern Illinois and northeastern Iowa is also undetermined. Mr. Oscar 

 Hershey has found water-bedded silt along the Pecatonica River and Yel- 

 low Creek valleys, in Stephenson County, Illinois, which he thinks imme- 

 diately preceded the loess in deposition. This silt is confined to low levels 

 along the valleys rising scarcely above the present water level. The full 

 extent of water action in this basin has not been determined. It seems 



