Xxii FISHES OF ILLINOIS 



concentric moraines with intervening 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 Shelby- 

 ville 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 have no relation to them. When the 

 Wisconsin streams broke through their confining moraines, they 

 had to find their way to the most accessible Illinoisan stream 

 as best they could. 



The present condition of the area of the Wisconsin drift 

 with its almost unbroken moraines, its black level prairies, peat 

 bogs, lake beds, shallow streams, and incomplete drainage is 

 believed to represent faithfully the condition of the Illinoisan 

 area at an earlier period in its history, and this correspondence 

 enables us to interpret many topographic relations in this area 

 which would not otherwise be apparent. For instance, the 

 control which the moraines of the Wisconsin area exercise on 

 the direction of its streams, the position and size of its lakes, 

 and the location and form of the tracts of black prairie soil are 

 very evident, and it is believed like control would be just as 

 evident in the Illinoisan area if the fragments of its moraines 

 were carefully studied and mapped so that they could be restored 

 and their influence shown. All that has been said about the 

 early history of the Illinoisan area applies as well to the Wis- 

 consin. The only material differences between them are due 

 to age and consequent degree of development. 



THE UNGLACIATED SOUTHERN AREA 



A natural division of this area would be into mountain 

 ridge and coastal plain. Regarding the first, little need be 

 mentioned beyond the facts that it is a true mountain in 

 structure, although its altitude is low (about 400 feet, on an 

 average, above the general level); that it is composed almost 

 entirely of limestones and sandstones of Mississipian; and that 

 it presents on its southern slope the only approach to volcanic 



