50 SOIL PHYSICS AND MANAGEMENT 



during which the surface loess was changed into soil which was 

 later buried in part by subsequent glaciers. The soils and peat beds 

 of the Peorian stage contain remains of cedar trees which grew in 

 the extensive swamps that existed at that time. 



(e) Early Wisconsin Glaciation, Loess and Interglacial 

 Stage. The Peorian stage was ended by another ice advance known 

 as the Early Wisconsin (Fig. 45), which came from the Labra- 

 dorean center of accumulation and formed a very extensive advance 

 reaching into Iowa, Illinois, Indiana, Ohio, Pennsylvania and cov- 

 ering practically all of New York and the New England states. 

 This glacier built up a system of moraines in the middle west that 

 is one of the most characteristic features. The terminal moraine 

 of the greatest advance is usually a distinct ridge. In Illinois and 

 Indiana it is known as the Shelbyville moraine. This glacier made 

 several advances and recessions, building up a moraine with each 

 advance, giving a series somewhat concentric with Lake Michigan 

 and other Great Lakes. A deposit of loess covers this drift in 

 Illinois and parts of Indiana to a depth of from three to six feet. 

 This glaciation was followed by a comparatively short unnamed 

 interglacial stage. 



(f) Late Wisconsin Glaciation. This stage was terminated 

 by an ice advance, the late Wisconsin, from all centers of accumula- 

 tion and in addition from many local centers. It was one of the 

 mpst extensive and uniform ice sheets during the entire glacial 

 period. The ice front did not extend southward as far as some 

 other advances except in Xew England, but there was probably a 

 solid ice front from the Atlantic to the Pacific. The erosive and 

 transporting power of the ice seemed to have been greatest at this 

 time, as is shown by the very high and characteristic moraines 

 formed near some of the Great Lakes. 



Incidental Features. Certain incidental features were de- 

 veloped in connection with the glaciers that served to modify the 

 soils in many regions. The drainage from the melting ice during 

 part of the time was entirely to the south. The streams were flooded 

 and overloaded with sediment, the deposition of which built up ter- 

 races of gravel, sand, silt, and even clay. When the glacier had re- 

 ceded so that the region in northern United States was partly cov 

 ered, the outlet of the lakes, which is naturally to the north and 

 northeast, was obstructed so that they overflowed the margin of the 

 basins and drained into the Mississippi River. Lake Agassiz, the 

 enlarged I^ake Winnipeg, is responsible for the soils of the Red River 



