XVI FISHES OF ILLINOIS 



their highest and lowest points, while variations of 300 to 400 feet 

 are often found. These differences, however, are not due to 

 variations in the general level, but to the presence of deep pre- 

 glacial valleys or of moraines, and often of both. 



For the present discussion the surface of the state may be 

 divided as follows : 



1. The northwestern unglaciated area. 



2. The areas of the Iowan and the Illinoisan drift. 



3. The area of the Wisconsin drift. 



4. The unglaciated southern area. 



THE NORTHWESTERN UNGLACIATED AREA 



It is believed that at one time the entire northern fifth of the 

 state was covered by rocks of the Trenton, Cincinnati, and Niagara 

 formations, these following each other from below upward in the 

 order named, and each covering the entire area. This portion of the 

 state became dry land at the close of the Silurian and was not 

 again submerged ; consequently, during the millions of years which 

 elapsed between its emergence from the ocean and the advent of the 

 first ice-sheet it was subjected to large erosion in spite of its low 

 relief. At some time during this long period a low arch was raised 

 across its northwestern corner, and here erosion became much more 

 effective than on the less elevated parts. 



The streams of that time cut for themselves canons 250 to 300 

 feet deep, extending entirely through the Niagara and Cincinnati, 

 but found their base level at or near the surface of the Trenton. 

 An extensive peneplain was formed at this .level, covering most of 

 the area now included in the nine counties which lie farthest west. 

 At various points over this peneplain, mostly in its northern and 

 western parts, fragments of the denuded strata were left in the form 

 of mounds which now rise above the general surface. Later the base 

 level was lowered and the rivers began again to deepen their chan- 

 nels, and they have continued this process until now they flow in 

 trenches cut in the rock often to a depth of 300 to 400 feet below the 

 general level. With the mounds rising above the general surface 

 and with the deep channels in which the rivers flow, the topography 

 of the country is extremely broken for that of the Mississippi 

 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 un- 

 touched. 



