210 GENERAL GEOLOGY. 



3. THE KEOKUK LIMESTONE. 



%w.-Lowee Archimedes Limestone of Owen and Others. 



Area and General Characters. The Keokuk limestone 

 consists in Iowa of about fifty feet in maximum thickness of 

 grayish limestone, usually having a bluish tinge, together 

 with about forty feet of shaly, calcareo-silicious strata, 

 usually containing many silicious geodes, and forming a 

 gradual passage to the St. Louis limestone above. It occu- 

 pies a more limited area than any other formation of the 

 Sub-carboniferous group in Iowa. It is well developed and 

 largely exposed at the city of Keokuk, from which the 

 formation derives its name, but the most northerly point 

 at which it has been recognized is in the northern part of 

 Des Moines county, where it has nearly thinned out, and 

 it appears also much thinner all along the eastern border 

 of its outcrop in that county than it does in the vicinity of 

 Keokuk. In the southwestern part of Washington county, 

 the St. Louis limestone appears in such proximity to the 

 Burlington limestone as to render it almost certain that the 

 Keokuk limestone has thinned out there. The general 

 westerly dip of the rocks in this part of the State is so 

 great, that the Keokuk limestone not only thins out upon 

 the Burlington limestone in Des Moines county, but it also 

 passes beneath the St. Louis limestone in its southwestern 

 portion. The general dip of all the formations, being 

 southerly as well as westerly, would naturally carry the 

 Keokuk limestone so far beneath the surface in Van Buren 

 county, that it would not appear at all there if it were 

 not that a gentle anticlinal axis, having a northerly and 

 southerly direction, brings it up to the surface again along the 

 Des Moines river at and in the vicinity of Bentonsport. From 

 this region the general dip carries it beneath the succeeding 

 strata, and it is seen no more in Iowa. It is only in the four 

 counties of Lee, Yan Buren, Henry, and Des Moines that the 

 Keokuk limestone is to be seen ; but it rises again and is seen 

 in the banks of the Mississippi river some seventy-five or 



