26 ILLINOIS STATE ACADEMY OF SCIENCE 



by a conspicuous brecciated layer in the midst of the St. 

 Louis limestone, but south of the city of St. Louis no break 

 in the record is recognizable. This mid-St. Louis with- 

 drawal, to a position as far south as Alton, was greater than 

 either of the preceding ones had been. 



At the close of St. Louis time the waters of the basin 

 were again withdrawn, and this withdrawal was even 

 greater than that of the mid-St. Louis, for the unconformity 

 between the St. Louis and the overlying Ste. Genevieve 

 limestone extends as far south as Ste. Genevieve, Missouri. 

 In the Ohio river sections of the Mississippian formations, 

 however, there is no stratigraphic break between these two 

 formations, a condition which shows that the waters of the 

 Illinois basin were not completely withdrawn from the 

 southern part of Illinois at this time. With the readvance 

 of the Ste. Genevieve sea, the waters again occupied much 

 of Illinois and spread v/estward, north of the Ozark land, to 

 a point at least as far west as Fort Dodge, lov/a which is 

 175 miles from the Mississippi river, and the waters may 

 have extended much farther than this for the record is now 

 completely buried beneath younger formations. 



With each one of these readvances of the waters in Lower 

 Mississippian time, the sea-pattern developed in the Illinois 

 basin must have been essentially similar in its general out- 

 line, although in detail undoubtedly there was much varia- 

 tion, but with the close of the Lower Mississippian, the 

 Iowa series, after the deposition of the Ste. Genevieve lime- 

 stone, the waters were more completely withdrawn than 

 they had been at any time since the opening of the Mississ- 

 ippian. This withdrawal may have been from the entire 

 continental area, although this cannot be certainly asserted. 



With the opening of Upper Mississippian or Chester 

 time, the oceanic waters again occupied the Illinois basin, 

 but the sea-pattern of this epoch was totally different from 

 that of Iowa time. During the Chester the embayment 

 never extended westward, north of the Ozark land, as it had 

 done in Iowa time, that land area being continuously con- 

 nected to the north, across Missouri and Iowa, with the 

 main land. The Illinois basin of Chester time was limited 



