CHAPTER II. 

 PHYSICAL FEATURES. 



Before entering upon a discussion of the glacial deposits, the leading 

 physiographic features of the region will be outlined. The region invaded 

 by the Illinois lobe occupies the midst of the great interior basin which on 

 the east rises to the Appalachian Mountains and on the west to the Rocky 

 Mountains. The State of Illinois has the distinction of being the lowest of 

 the North-Central States. Its mean elevation has been estimated by Mr. 

 Henry Gannett to be about 600 feet above tide, while that of Indiana is 

 700 feet, Michigan 900 feet, Wisconsin 1,050 feet, Iowa 1,100 feet, and 

 Missouri 800 feet. 1 



Including the south end of the Lake Michigan basin, the region cov- 

 ered by the Illinois lobe shows a range of not far from 1,200 feet, the 

 bottom of the lake opposite Racine, Wisconsin, being at sea level, while 

 the highest rock surfaces of Illinois covered by glacial drift are nearly 

 1,200 feet above sea level. It is not unlikely that glacial erosion and differ- 

 ential depression have increased this range, but there is reason to think that 

 in preglacial times the lake basin was at least several hundred feet lower 

 than the highest ridges of the border districts and that it had a controlling 

 influence upon the course of the ice movement. 2 



The State of Illinois has been covered by a careful barometric survey 

 conducted by Prof. C. W. Rolfe, of the University of Illinois, a survey 

 which had for its object the preparation of a topographic model of the State 

 for the Columbian Exposition. Professor Rolfe used as datum points the 

 altitudes of railway stations which had been determined by surveyor's level. 

 These are found at intervals so frequent in nearly every county of the State 

 that there is but little room for error in his maps. He has exercised great 



'The average elevation of the United States, by Henry Gannett: Thirteenth Ann. Rept. U. S. 

 Geol. Survey, 1892, p. 289. 



2 See discussion by R. D. Irving: Geol. of Wisconsin, Vol. II, 1877, pp. 632-634. Also T. C. 

 Chamberlin: Ann. Rept. Wisconsin Geol. Survey, 1878, pp. 23-32. 



