xx FISHES OF ILLINOIS 



moraines, and so these areas gradually came to be covered with 

 belts of timber, between which were the lakes or marshes which 

 afterward became prairies. 



As the lakes gradually became marshy, the water, flowing 

 from one to the other through the concentric moraines, sought 

 the lowest channels and formed continuous streams. Since 

 certain of the preglacial channels were not completely filled 

 with drift throughout their entire lengths they offered depressions 

 here and there, and the streams followed their course for con- 

 siderable distances, so that in the end the general direction of 

 the stream was often largely controlled by these valleys. 



As time went on these main streams threw off branches 

 behind the moraines which in their turn divided and subdivided, 

 each little branch pushing its channel back towards the nearest 

 slough. In this way a complete drainage system was gradually 

 established, but the courses of the larger branches, and many 

 of the smaller as well, were largely controlled by the moraines 

 behind which they were developed. Gradually, and long before 

 the drainage system was complete, those branches which were 

 pushing backward toward the moraines united with the flood- 

 water streams which flowed down their sides and began to eat 

 into the moraines themselves, thus dividing them into series of 

 isolated hills and short ridges which we now find scattered all 

 over this area. In some cases they removed the moraines 

 entirely. Only a few of these old morainic systems have been 

 studied and are shown on the accompanying map (III.), but 

 many others are known to exist. 



The above is, in brief, the history of this area, and indicates 

 in a general way how its streams and surface features were 

 formed. As the drift was deposited on an irregular surface its 

 depth varied greatly, and in many places the streams have cut 

 entirely through it, alternately crossing the divides and channels 

 of former streams, and consequently flowing now on rock and 

 now on mud beds. 



With the establishment of a drainage system, erosion of the 

 prairies began, and every storm since that time has carried away 

 portions of the black prairie soil, until now, in many places, it has 

 nearly or entirely disappeared, leaving the gray to brown, more 

 or less acid, subsoil at the surface. In the lake beds, which 

 were protected from erosion, the black soil has been retained 

 and, in some places, even thick beds of peat have been formed. 

 Some lakes were so situated that streams flowing into them 



