THE TOPOGRAPH* AND HYDROGRAPHY OF ILLINOIS XXI 



brought quantities of sediment. The coarser particles, or sands, 

 were deposited as soon as the velocity was checked, but the 

 water in the lake was kept in motion sufficiently rapid so that 

 the finer sediment was not dropped, but carried away. In this 

 manner the beds of the lakes were covered with thick layers of 

 sand. When drainage was established, this sand, then left dry, 

 was heaped by the wind into dunes and hills. Illustrations of 

 this may be found in the Winnebago swamps, the sandy areas 

 of Mason, Kankakee, and Tazewell counties, and in many other 

 places. 



After an interval covering thousands, perhaps tens of 

 thousands, of years following the retreat of the earlier ice-sheets, 

 the northeastern portion of the state was again covered by ice. 

 As this ice melted, its outwash deposited here and there over 

 the older drift a layer of fine but well-assorted material called 

 loess. After the ice had disappeared and the climate had be- 

 come less humid, this loess was rearranged by the wind and 

 quite probably received additions of similar wind-borne material 

 from the western plains. We speak of it as having been derived 

 from the Iowan and Wisconsin glaciers, but it is quite certain 

 that at least some of these deposits were formed during the 

 retreat of the Illinoisan ice, and rearranged and redistributed 

 by wind during the great drouth which covered part of the 

 interval between the earlier and later invasions. Most of the 

 loess in this state is formed in a broad belt following roughly the 

 course of the Mississippi and its larger tributaries. 



All the elements whose origin is here indicated enter into the 

 surface of the area now under discussion at various points. The 

 exact location of many of them will be mentioned in connection 

 with the description of the various river systems. 



THE AREA OF THE WISCONSIN DRIFT 



As stated above, long after the retreat of the earlier glaciers 

 the northeastern corner of the state was invaded by a new ice- 

 sheet called the Wisconsin glacier. It covered this portion of 

 the state as far south as Paris and Shelby ville, leaving, when it 

 retired, a prominent moraine which runs through these places 

 and then turns northward, passing near Decatur, Clinton, Pekin, 

 Princeton, Sycamore, and Harvard, as shown on the accompany- 

 ing map (III.). This ridge is known as the Shelby ville or 

 Mattoon moraine. In its retreat this glacier left a series of 



