XVlll FISHES OF ILLINOIS 



this does not mean that it presents no marked variations. Few 

 of the 102 counties in the state have a difference of less than 150 

 feet between 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 preglacial 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 lowan 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, Maquoketa, and 

 Niagara formations, these following each other from below up- 

 ward 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, except possibly in small 

 areas and for brief periods; 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 cafions 250 to 

 300 feet deep, extending entirely through the Niagara and 

 Maquoketa, but found their base level at or near the surface of 

 the Galena. 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, 

 mostlj^ 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 channels, 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 



