Xviii FISHES OF ILLINOIS 



their entire lengths they offered depressions here, and there, and the 

 streams followed their course for considerable distances, so that in 

 the end the general direction of the stream was often largely con- 

 trolled 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 channels 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 pro- 

 tected 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 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 drop- 

 ped, 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 area of Mason and Tazewell counties, and in many other 

 places. 



