4IO STUART WELLER 



makes the lithologic separation of the BurHngton and Keokuk 

 somewhat difficult. 



In southeastern Iowa the land-derived sediments became more 

 dominant in Warsaw time, although there is no stratigraphic break 

 between the Keokuk and the Warsaw. Farther south in the Mis- 

 sissippi Valley basin, limestone deposition continued into Warsaw 

 time, and in southwestern Missouri shale deposition of this age 

 is absent or practically absent throughout the entire epoch. 

 During Warsaw time, however, the northern shore of the basin 

 continued its southward migration and by mid-Warsaw time it 

 doubtless occupied a position somewhere between the southern 

 border of Iowa and St. Louis, Missouri, and at this time the 

 clastic sediment extended as far south as southeastern Missouri. 

 The lower Warsaw only, therefore, was deposited in southern 

 Iowa. 



During Spergen time the sea readvanced to the north and 

 again occupied what is now southeastern Iowa, where the Spergen 

 limestone rests unconformably upon the Warsaw and where 

 locally much or all of the lower Warsaw that had been deposited 

 was eroded during the time when the shore line occupied a more 

 southern position and when this part of Iowa was an area of dry 

 land. At St. Louis and to the south the Warsaw sedimentation 

 passed without interruption into the Spergen, but with the north- 

 ward shifting of the shore line of the basin and the consequent 

 greater remoteness of the region from the source of land wash pure 

 limestone sedimentation without clastic materials of any sort was 

 reinitiated. 



During the later portion of Spergen time the shore line again 

 shifted to the south, and must have occupied about the position 

 of the late Warsaw land margin. This was followed by another 

 shift to the north, which is evidenced by the unconformable 

 relations of the lower St. Louis in Iowa, resting upon the eroded 

 surface of the Spergen, the erosion in places having even cut 

 through the Spergen, so that the higher formation rests upon beds 

 of Warsaw age. Another oscillation of the shore line occurred in 

 mid-St. Louis time, for the upper St. Louis beds in Iowa are 

 separated by a distinct erosional unconformity from the lower St. 



