THE CHESTER SERIES IN ILLINOIS 409 



beds at Burlington, Iowa, the blue shales beneath the Chonopectus 

 sandstone, are continuous with the Sweetland Creek shales which 

 commonly have been placed in the Upper Devonian. However, 

 the Sweetland Creek shales are unconformable upon the beds 

 beneath them, and it is possible that the whole of the Sweetland 

 Creek shale should be placed in the Mississippian. This con- 

 dition of unconformity of the Kinderhook upon underlying for- 

 mations of various ages persists entirely around the Ozark region 

 of Missouri and northern Arkansas. This situation shows that 

 preceding Kinderhook time the waters were withdrawn from the 

 Mississippi Valley basin, at least north of the Ohio River, and 

 possibly were withdrawn from the entire continent. The details 

 of the first advance of the Mississippian waters cannot be discussed 

 in this place, but the sediments recording this epoch are exceed- 

 ingly various in character, there being sandstones, shales, and 

 hmestones. As the waters advanced the Ozark land mass was at 

 last largely or perhaps wholly submerged, and the mainland shore 

 line of the Illinois basin crossed northern Illinois or southern 

 Wisconsin and continued westward into Iowa. The position of 

 this shore Kne was certainly north of Chicago, as is evidenced by 

 the typical Lower Burlington fauna which has been recorded from 

 the northern part of that city.' At this time the waters covering 

 southeastern Iowa, the Ozark region of Missouri, and the adjacent 

 parts of Illinois were quite free from land wash, and the very pure 

 Burlington limestone was being deposited throughout this region. 

 During Keokuk time there was a shifting of the northern shore 

 line in a southerly direction, to such a position that a certain 

 amount of clastic material was washed into that part of the sea 

 which covered southeastern Iowa and the adjacent parts of Missouri 

 and Illinois. The presence of this land wash is shown in the shaly 

 beds which are present in the more northerly Keokuk exposures, a 

 lithologic character which differentiates the Keokuk from the 

 underlying Burhngton limestone. Farther south in Illinois and 

 in Missouri, both in the' southeastern and southwestern parts 

 of the state, at a greater distance from the shore line, these 

 clastic beds are not present in the Keokuk, a condition that 

 ' Davis, Jour. GeoL, Vol. XXV (191 7), pp. 576-83. 



