THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS XV 



The Topography and Hydrography of Illinois* 



By Charles W. Rolpe, M. S. 



The State of Illinois may be described as a great plain sloping 

 gently towards the south, the northernmost fifth of which is under- 

 laid by rocks of Silurian age, while the surface rocks of the remaining 

 four fifths are the limestones, sandstones, and shales of the sub- 

 carboniferous and coal measures. 



The highest portion of this plain lies in -the northern part of Jo 

 Daviess and Stephenson counties, where the general surface has an 

 elevation of something over 1,000 feet, and mounds rise more than 

 200 feet above this level. The highest point is Charles Mound, near 

 the Wisconsin line, which is 1,257 feet above the sea. From this 

 point the surface slopes rather rapidly to the east and south, declin- 

 ing to an average altitude of about 800 feet in Lake county and of 

 700 feet in Whiteside county. South of Whiteside county the 

 surface levels across the state from east to "west are essentially the 

 same wherever the line is drawn, but southward the surface slopes 

 gradually until an average level of 400 feet >is reached just north of 

 the Ozark ridge. This ridge is an eastern extension of the Ozark 

 Mountain range, whose highest peaks in Illinois are Williams Hill, 

 in Pope county, which reaches an elevation of 1 ,046 feet, and Bald 

 Knob, in Union county, 985 feet high. The average altitude of the 

 ridge is from 750 to 800 feet. South of it the surface slopes rapidly 

 to the low valley of the Cache River, the general altitude of which 

 does not exceed 325 to 350 feet. The lowest point in the state is 

 at Cairo, where low water on the Ohio River is 268 . 58 feet above the 

 sea. 



While the general surface of the state is unusually level, 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 



*The general system of the hydrography of the state is so largely a consequence 

 of its surface geology that it can be clearly understood only by way of its geological 

 antecedents and relations. For this reason Professor C. W. Rolfe, now and for 

 many years head of the Department of Geology in the University of Illinois, was 

 asked to prepare this chapter. With his discussion has been incorporated, with 

 his approval, some additional matter relating especially to the waters themselves, 

 dompiled from field notes of the State Laboratory, and from more general sources. — 

 S. A. Forbes. 



