THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS XVH 



THE AREAS OF lOWAN AND ILLINOISAN DRIFT 



Before describing these areas it is well to call attention to the fact 

 that the preglacial drainage of the entire state seems to have been 

 from northeast to southwest, and that while most of these early 

 stream beds were completely filled by the drift from the ice-sheets, 

 some of them were so large and deep that they were not entirely 

 filled throughout their length, and now control the general direction 

 of our larger streams. Probably, however, no one of them follows 

 a preglacial channel throughout its entire length, and nearly all of 

 the smaller streams flow in postglacial channels, the courses of 

 which have been largely determined by moraines. 



Coming now to the areas mentioned in the last heading, it is 

 believed that all of that part of the state which lies north of the 

 Ozark ridge, with the exception of the extreme northwest corner, 

 was covered by one or more of the earlier ice-sheets, and that, when 

 these retreated, they left behind them a thick sheet of drift which 

 filled the smaller channels completely, and some portions of the 

 larger ones as well. Upon the general surface thus formed they 

 also laid down ridges of drift which extended across the country, 

 forming effective dams to the drainage. These dams, which are 

 called moraines, varied in height from a few feet to a hundred or 

 more, and from a few rods to one or several miles in width. They 

 were generally concentric, and so lay nearly parallel to each other. 

 When they were far apart they inclosed large areas which had no 

 outlets, and, filled by rains, formed extensive lakes; but when 

 they were close together the intervening lakes were necessarily 

 smaller and more numerous. The water supply of the time greatly 

 exceeded evaporation, and so these basins were soon filled to the 

 brim and overflowed at the lowest points of the moraines which 

 surrounded them. These openings gradually deepened. Ulti- 

 mately, by the lowering of their outlets, and also by filling with 

 deposits, the lakes were converted into marshy plains or prairies. 



During the time in which the lakes were in existence nothing 

 prevented the growth of vegetation on the confining 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 pre- 

 glacial channels were not completely filled with drift throughout 



(b) 



