THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS xix 



After an interval covering thousands, perhaps tens of thousands, 

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

 eastern 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 become 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 Shelbyville, leaving, when it retired, a 

 prominent moraine which runs through these places and then turns 

 northward, passing near Decatur, Clinton, Pekin, Princeton, Syca- 

 more, and Harvard, as shown on the accompanying map (III.). 

 This ridge is known as the Shelbyville or Mattoon moraine. In its 

 retreat this glacier left a series of concentric moraines with interven- 

 ing lake-beds, the larger of which are well shown on the map. 



Another fact, also partially indicated on the map, is that the 

 drainage system in the part of the state north and east of the 

 Shelbyville moraine is not nearly so well developed as in the older 

 Illinoisan drift area, and consequently the streams do not have so 

 many branches. As the streams break through the Shelbyville 

 moraine, they often change the direction of their courses entirely, 

 thus forming curious curves. This is doubtless due to the fact that 

 as the Wisconsin drift sheet is superimposed on the Illinoisan drift, 

 the beds of the streams developed on the surface of the latter are 

 continued under the former, while the streams on the Wisconsin 



