P-\PERS OF GENER.\L INTEREST 25 



Ozark land which was sufficiently submerged to prevent any 

 transportation of land detritus by the streams into the sur- 

 rounding oceans. It was at this time that the Spergen lime- 

 stone was being laid down, a formation which is a nearly 

 pure limestone at most localities. In southeastern Iowa 

 this formation lies unconformably upon the underlying beds, 

 but in the southern portion of the basin there is no e\idence 

 of any interruption in sedimentation in passing from the 

 Warsaw to the Spergen. If the exact southern limit of the 

 condition of unconformity could be determined, the exact 

 location of the southernmost position of the shore line in 

 late Warsaw time could be established. 



After Spergen time the waters of the Illinois basin again 

 retreated southward to a position essentially the snme as 

 that of the pre-Spergen retreat. The evidence for this 

 shifting of the shore line is exhibited in the uncomformable 

 contact of the St. Louis limestone which lies next above the 

 Spergen, or above whatever formation is subjacent. During 

 the post-Spergen retreat the erosion of the surface of what 

 is now southeastern Iowa was unequal, in places the whole 

 of the Spergen was removed, while elsewhere a greater or 

 less thickness of the formation remained. Where the Sper- 

 gen was wholly removed the St. Louis limestone rests di- 

 rectly upon the Warsaw beds; elsewhere it rests uncom- 

 formably upon the Spergen. In southern Illinois the con- 

 tinuity of sedimentation from Spergen to St. Louis time was 

 not interrupted, a condition indicating the continuous occu- 

 pation of that portion of the Illinois basin by the ocean 

 waters. The southern extent of the post-Spergen uncoi- 

 formity seems to be essentially the same as that of the post- 

 Warsaw, and so far as can be determined from data now 

 available, the shore lines of these times occupied about the 

 same positions. 



Again a fluctuation of the waters of the basin took place in 

 the midst of the period of deposition of the St. Louis lime- 

 stone, this v.'ithdrawal, followed by a readvance, being 

 proven by a stratigraphic break in southeastern Iowa with 

 the upper division of the St. Louis limestone resting un- 

 comformably upon the lower portion of the same formation. 

 This interruption is exhibited as far south as Alton, Illinois, 



