HISTORICAL 315 



mained practically unchanged until after the limestone of the Platy- 

 merella manniensis zone was deposited, after which there occurred move- 

 ments of sufficient importance to materially change the outlines of these 

 basins. If this interpretation is correct, the most important movements 

 of this time in the Mississippi Valley occurred immediately after the 

 deposition of the limestone of the Platymerella manniensis zone, instead 

 of just before. For this reason, it is here proposed to shift the upper 

 boundary of the Edgewood formation and the basal part of the Sexton 

 Creek limestone 3 or 4 feet higher than formerly, placing it at the top 

 of the limestone containing Platymerella manniensis in Illinois and 

 Missouri, instead of at the base of this zone, where it has previously been 

 drawn. 



Deposition of the limestone containing Platymerella manniensis was 

 followed by crustal movements, which changed in a very important way 

 the outlines of the basins in which the Edgewood strata were accumulated. 

 Disturbances in the Ozarkian region resulted in the formation of an arch 

 trending toward the northeast nearly through the present site of Saint 

 Louis. This arch formed a barrier to the advance of the southern sea, 

 not only during all of Sexton Creek time, but it remained effective during 

 the subsequent times of submergence of this region throughout the re- 

 maining part of the Silurian and all of the Lower and Middle portions 

 of the Devonian period. The Sexton Creek sediments that accumulated 

 in the basin south of this barrier are well exposed in Alexander and Union 

 counties, in southwestern Illinois, where they have an aggregate thickness 

 of about 70 feet. 



Nearly or quite coincident with the Ozarkian movement there occurred 

 warping in the northern Illinois-southern Wisconsin area, which sub- 

 merged the barrier that existed there in Edgewood time and permitted 

 the sea from the Gulf of Saint Lawrence region to extend toward the 

 west and south as far as Calhoun County, in western Illinois. g In this 

 northern basin the Stricklandinias flourished in great numbers and variety 

 near the middle of Sexton Creek time. During the same time in the 

 southern basin in Illinois and in the Brassfield basin of Ohio and Ken- 

 tucky the only known species of that genus was Stricklandinia triplesiana, 

 which also occurs associated with typical forms of Tnplecia ortoni in the 

 middle part of the Sexton Creek limestone along Sexton Creek, in south- 

 ern Illinois, from which creek the name of the formation was taken. To 

 the rocks in this southern basin that are equivalent in age to the Brass- 

 field strata of Ohio and Kentucky it is proposed to restrict the name 

 Sexton Creek limestone. The strata of corresponding age that accumu- 

 lated in the northern basin, including western Illinois and eastern Mis- 



