VOLUME XXVIII NUMBER 4 



THE 



JOURNAL OF GEOLOGY 



MAY-JUNE igzo 



THE CHESTER SERIES IN ILLINOIS' 



STUART WELLER 

 University of Chicago 



PART I 



The Mississippian rocks in Illinois occupy three distinct areas 

 along the western and southern borders of the state. The northern- 

 most of these areas is the larger, and extends from southern Mercer 

 County on the north to northern Madison County on the south. 

 Throughout this entire distance, except for an interval in Pike, Cal- 

 houn, and Jersey counties, where older rocks are exposed, the rock 

 formations of the Mississippian system constitute the Mississippi 

 River bluffs. This area also includes the Mississippian strata 

 which are exposed in the valley of the Illinois River as far north as 

 Scott, Brown, and Schuyler counties. Nowhere in this area do the 

 higher formations of the system occur, the youngest formation 

 exposed being the Ste. Genevieve limestone, in the summit of the 

 bluffs above Alton. 



The second of the three areas occupies portions of St. Clair, 

 Monroe, Randolph, and Jackson counties. This area includes 

 about 85 miles of the Mississippi River bluffs from a short distance 

 below East St. Louis to the gap formed by the valley of the Big 

 Muddy River, and at only one locaHty in this entire distance, at 



' Published by permission of the Directors of the Geological Surveys of Illinois 

 and Missouri. 



