THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS XV11 



The Topography and Hydrography of Illinois* 



By Charles W. Rolfe, M. S. 



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

 gently towards the south, the northernmost fifth of which is 

 underlaid by rocks of Silurian age, while the surface rocks of 

 the remaining four-fifths are the limestones, sandstones, and 

 shales of the Mississippian and Pennsylvanian, with small areas 

 of Ordovician, Silurian, and Devonian. 



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 1,241 feet 

 above the sea. From this point the surface slopes rather rapidly 

 to the east and south, declining 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 1,065 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, 



*The general system of the hydrography of the state is so largety a consequence of its sur- 

 face geology that it can be clearly understood only by way of its geological antecedents and 

 relations. For this reason Professor C. W. Rolfe, for many years head of the Depart- 

 ment 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, compiled from field notes of the State Laboratory, and from more 

 general sources. — S. A. Forbes. 



—2 F 



