PAPERS OF GENERAL INTEREST 21 



SOME EVENTS IN THE GEOLOGICAL HISTORY OF 

 SOUTHERN ILLINOIS 



Prof. Stuart Weller. University of Chicago 



From a geological point of view southern Illinois, along 

 with southeastern Indiana, occupies a basin lying between 

 the Ozark region of Missouri on the west, and the Cincin- 

 nati region on the east. Throughout geological time these 

 two areas have been positive areas, that is regions which 

 have had a tendency to be uplifted at intervals to a greater 

 degree than their surroundings. At times they have been 

 islands completely encircled by the waters of the ancient 

 seas, at other times they have been covered by waters of 

 much shallower depth than the adjacent areas. 



In discussing the geological history of this area, as of 

 any region, the geologist must draw his inferences concern- 

 ing the succession of events from his field observations upon 

 the rock strata of the earth's crust. Since geological time 

 is inconceivably long, and since the complexity of geological 

 history is exceedingly great, a consideration of the entire 

 course of the geological history- of southern Illinois would 

 consume time far beyond that at my disposal this afternoon. 

 I will, therefore, confine my remarks to a comparatively 

 short time interval, namely that beginning with the Mississ- 

 ippian period and extending to the close of the Paleozoic era. 



Rock strata of Mississippian age occupy a belt, including 

 the Mississippi river bluffs and the adjacent region to the 

 east for a distance of 15 miles or less, extending from a 

 point in St. Clair County south of East St. Louis, to the 

 valley of Big Muddy river. Strata of the sam.e age occupy 

 another belt extending eastwardly across Union, Johnson, 

 Pope and Hardin Counties, from the Mississippi river to 

 the Ohio, south of the Ozark upland which occupies the 

 northern portion of the counties mentioned. 



Throughout Mississippian time the southern Illinois basin 

 was occupied by a great gulf-like emba\Tnent of the ocean 

 which opened to the south ; it was in a way an extension to 

 the north of the ancient Gulf of Mexico. This embayment 

 extended far beyond the limits of the present outcrops of 



