PHYSICAL FEATURES OF THE REGION. 



9 



possibly it would not exceed 500 feet above tide. This low altitude extends 

 eastward some distance into Indiana. The low altitude of this district, as 

 well as that of the Lake Michigan basin, as noted above, probably influ- 

 enced the ice flow and invited its great southward extension in the State of 

 Illinois. 



The following table showing the range in altitude of the drift surface 

 and of the rock floor in each county of Illinois serves to show not only the 

 general range but also the variations within small districts. As the impres- 

 sion prevails widely that Illinois is a very level region, the table may be of 

 service in dispelling this illusion. The impression of flatness arises from 

 the general absence of abrupt reliefs. A comparison of the ranges in 

 altitude in the several counties shows that 18 counties have a range of over 

 400 feet in their drift surface and that 22 show an equally great range in 

 rock surface. In 55 counties, or more than half the number in the State, 

 there is a range of less than 300 feet in the drift surface, while in 62 coun- 

 ties the known range in rock surface falls below 300 feet. In the remaining 

 29 counties the range in altitude of drift surface falls below 200 feet. 



Table showing altitudes of drift surface and rock floor, with their range, in each county 



of Illinois, (a) 



Highest 

 point. 



Lowest 

 point. 



Lowest 



known rock 



surface. 



Range in 

 drift sur- 

 face. 



Range in 

 rock sur- 

 face. 



Jo Daviess . . 

 Stephenson. . 

 Winnebago. . 



Boone 



McHenry 



Lake 



Cook 



Dupage 



Kane 



Dekalb 



Ogle 



Lee 



Carroll 



Whiteside . . . 

 Eock Island. 



Mercer 



Henry 



Bureau 



1,257 



1,170 



990 



1,040 



1,040 



913 



910 



812 



975 



950 



1,000 



1,017 



1,020 



890 



840 



857 



850 



987 



1,257 



1,170 

 975 



1,000 

 875 

 607 

 699 

 680 

 760 

 813 



1, 000 

 875 



1,000 

 850 

 675 

 736 

 815 



575 

 735 

 680 

 720 

 740 

 581 

 581 

 581 

 614 

 649 

 660 

 649 

 568 

 564 

 530 

 523 

 562 

 431 



453 



(500) 

 (425) 

 (450) 

 580 

 505 

 478 

 581 

 579 

 542 

 (400) 

 (400) 

 435 

 435 

 (425) 

 (385) 

 560 

 330 



435 

 310 

 320 

 300 

 332 

 329 

 231 

 361 

 301 

 340 

 360 

 452 

 326 

 310 

 .334 



804 



(670) 



(550) 



(550) 



295 



102 



221 



99 



181 



271 



(600) 



(475) 



565 



415 



(250) 



(351) 



255 



470 



a Altitudes of rock surface in parentheses have heen inferred from known altitudes in bordering counties. 



