8 THE ILLINOIS GLACIAL LOBE. 



care in reducing- to a minimum errors arising from barometric fluctuations, 

 He lias kindly allowed the writer to trace from his unpublished map sheets 

 such contours as are represented on the accompanying map (PL III). In 

 the hilly-driftless tracts in the northwest corner, and in the southern end of 

 the State, the surface is so uneven that only 100-foot contours are intro- 

 duced. But the altitude of the greater part of the State is represented by 

 contours with 50-foot interval. For very small areas, covering but a frac- 

 tion of a square mile, the contours must necessarily be omitted on a map of 

 so small scale. A few such areas occur in the hilly districts in the north- 

 western and the southern portions of the State. In PL IV the areas 

 between each 100-foot contour are shown in color, and the bottom of Lake 

 Michigan is also represented. 



In a general way the altitude decreases from north to south in the 

 State of Illinois, there being four counties on the north border (Jo Daviess, 

 Stephenson, Boone, and McHenry) in which points rise 1,000 feet above 

 tide. Near the southern border of the Coal Measures basin the average 

 altitude is below 500 feet. There is, however, south from the Coal Meas- 

 ures basin a prominent ridge which rises nearly to the altitude of the 

 northern portion of the State, its crest reaching at one point an altitude 

 of 1,047 feet (Rolfe). A reference to Pis. Ill and IV will make clear 

 the altitudes and slopes of Illinois and also of the portions of southwestern 

 Michigan and western Indiana embraced in this discussion. The highest 

 point in Illinois (1,257 feet) is Charles Mound, on the Illinois-Wisconsin 

 line, in the northwest county. The lowest points are near the junction of 

 the Ohio and Mississippi, and fall slightly below 300 feet at low-water 

 stages of the stream. At high-water stages none of Illinois is below 300 

 feet. It appears from the table below (p. 12) that only 125 square miles, 

 or an area of less than four townships, rises above the 1,000-foot contour, 

 and that only 10,747 square miles, or less than one-fifth of the State, falls 

 below the 500-foot contour. About 20,000 square miles, or more than 

 one-third of the State, stands at 600 to 700 feet above tide, or at about the 

 average altitude of the State. 



The thickness of the drift is so great in the northeastern fourth of Illinois 

 as to convey a false idea of the altitude of the rock surface in that region. 

 Were the drift coating entirely removed, the average elevation would prob- 

 ably be as low as the surface of Lake Michigan (580 feet above tide), and 



