THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS xix 



Valley. Much of the irregularity shown in Jo Daviess county, 

 however, was produced during and since the glacial period, for 

 the ice-sheet which advanced on the state from the north was 

 divided in southern Wisconsin and left this part of our state 

 untouched. 



THE AREAS OF IOWAN 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 numer- 

 ous. The water supply of the time greatly exceeded evapora- 

 tion, and so these basins were soon filled to the brim and over- 

 flowed at the lowest points of the moraines which surrounded 

 them. These openings gradually deepened. Ultimately, 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 



