THE CHESTER SERIES IN ILLINOIS 41 1 



Louis, and this stratigraphic break is probably exhibited in the 

 very greatly brecciated zone in the midst of the St. Louis Kmestone 

 as far south as Alton, IlKnois.' 



In the region south from St. Louis, during all this time, the 

 sedimentation had been continuous with no interruption whatso- 

 ever, showing that the Mississippian sea had continually occupied 

 this southern portion of the basin. At the close of St. Louis time, 

 however, a greater oscillation in the sea-level occurred, and the 

 waters were withdrawn to some extent from the Ozark land as 

 well as from Iowa and from all of the region between Iowa and 

 Ozarkia. This withdrawal was only temporary, however, for the 

 sea reoccupied the entire region and in it the Ste. Genevieve lime- 

 stone was deposited unconformably upon the St. Louis. This 

 sub-Ste. Genevieve unconformity is well exhibited near Ste. 

 Genevieve, Missouri, and wherever the Ste. Genevieve limestone 

 is present to the north of this locaKty to its most extreme northern 

 outcrops in the Des Moines Valley of Iowa, near Fort Dodge. 

 In a southeasterly direction from Ozarkia, however, in Hardin 

 County, Illinois, sedimentation was continuous from the St. 

 Louis into the Ste. Genevieve limestones, with no interruption of 

 any sort, showing that the Mississippian sea continuously occupied 

 this portion of the Illinois basin. The record of the extension of 

 the Ste. Genevieve sea to the north and west of Ozarkia is obscured 

 by the presence of younger sediments, but it is not unlikely that 

 this ancient island was entirely surrounded at this time. Follow- 

 ing Ste. Genevieve time the sea withdrew completely from the 

 Illinois basin. Wherever the Chester formations are present their 

 faunas appear abruptly, and wherever the sections have been 

 examined critically there is evidence of unconformity between 

 them and the underlying formations. 



In summing up the history of the IlHnois basin in Lower 

 Mississippian time, the succession of events consists of oscillatory 

 movements of the sea occupying the basin, these oscillations being 

 exhibited by the shifting of the bounding shore line of the basin 

 from north to south and back again to the north. The greatest 



' These statements concerning the stratigraphic relations in southeastern Iowa 

 are based chiefly upon the field observations of Dr. J. M. Van Tuyl. 



