The Fort Payne Formation (Valmeyeran, Mississippian) in the Illinois basin in part grades laterally into the Ullin Limestone and in part thins and pinches out under an increasing thickness of the Ullin. The Fort Payne is a deep-water basin facies consisting of dark colored, siliceous, sparsely fossiliferous, micritic limestone. This facies grades laterally into a deep-water shelf facies of the Ullin composed of light-colored, fine- to coarse-grained, crinoid- and bryozoan-rich bioclastic limestone. In Lawrence and Wabash Counties, Illinois, the shelf-basin transition occurs along the western edge of the La Salle anticline. Significant shelf-basin facies changes also occurred in this same geographic area during Silurian and Devonian deposition.

The depositional unit containing the Fort Payne facies thins westward and pinches out at places in Hamilton and Wayne Counties, Illinois. There it is overlain by several hundred feet of light-colored bioclastic Ullin Limestone that is younger than the part of the Ullin that grades laterally into the Fort Payne facies.

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