24 ILLINOIS STATE ACADEMY OF SCIENCE 



westward across Iowa and probably reached to the Rocky 

 Mountain region. Cincinnatia was a low lying island or 

 shoal water area. During this time, with no immediate 

 source for terrestrial materials, the sediments accumulating 

 over southern Illinois were largely calcium carbonate of or- 

 ganic origin, along with considerable quantities of colloidal 

 silica which was precipitated from fresh waters entering 

 the basin from rivers. Near the shore line clastic deposits 

 must have been accumulating, but they did not reach to the 

 area now occupied by southern Illinois. 



After Burlington time the waters of the Illinois basin 

 underwent a number of withdrawals and readvancements, 

 but the waters of the basin probably had their greatest ex- 

 tension during the Burlington epoch. The Keokuk epoch was 

 initiated by partial withdrawal of the waters of the basin, 

 the northern shore line coming to occupy such a position 

 that terrigenous material in the form of fine mud was some- 

 what extensively deposited as shale beds, as far south as 

 southeastern Iowa and the adjacent parts of Missouri and 

 Illinois, while in Burlington time the sedimentary accumu- 

 lation in these same areas was wholly organic in origin. 

 That part of the basin which is now southern Illinois, how- 

 ever, still remained at such a great distance from the shore 

 lines that limestone accumulations continued uninterrupt- 

 edly through the Burlington and Keokuk time, making it 

 more difficult to separate the strata of these two epochs. In 

 Warsaw time the shore line shifted still farther to the south, 

 and by mid-Warsaw time it occupied a position somewhere 

 between the southern border of Iowa and the city of St. 

 Louis. At this time also, the ocean level was sufficiently 

 lowered to permit the streams of Ozarkia to carry terri- 

 genous material into the sea, so that the Warsaw formation 

 of southern Illinois contains notable shale deposits, espec- 

 ially in the Mississippi river sections, although in southern 

 Illinois, at a greater distance from the shore line, the sedi- 

 mentation was continuously limestone. 



Following Warsaw time there was a readvance of the 

 northern shore line until the waters of the Illinois basin 

 spread again into northern Illinois, and westward into Iowa 

 for an unknown distance, and completely surrounded the 



