Rocks of Mississippian age cropping out in the northwestern Highland Rim include, in ascending order, the Maury Formation, New Providence Shale, Fort Payne Chert, Warsaw Limestone, St. Louis Limestone, and Ste. Genevieve Limestone. Field and microscopic examination of the post-New Providence formations shows that each contains characteristic and lithologically distinct units reflecting variations in depositional environment. The Fort Payne is characterized by an abundance of chert which occurs either as discrete beds separated by layers of highly siliceous limestone or as very rough, irregular plates intimately associated with limestone. In both types of chert the silica is largely cryptocrystalline and probably was syngenetically concentrated from silica-rich calcareous ooze. Coarse-grained fossil-fragmental calcarenite is the dominant lithology of the Warsaw. Subordinate lithologies include fine-grained siliceous calcarenite, coquinite, and fine-grained dolomitic limestone. Clear secondary calcite is generally lacking in the the Warsaw and much of the matrix in the calcarenite and coquinite appears to have developed through recrystallization. Most of the chert in the Warsaw is clearly of secondary origin. Like the Warsaw, the St. Louis consists primarily of coarse-grained fossil-fragmental calcarenite, but it differs in that much of the matrix is clear secondary calcite. Other lithologies in the St. Louis are coquinite, foraminiferal limestone, and siliceous and dolomitic limestone. Beds of shaly limestone also are present locally. The Ste. Genevieve Limestone, which is of limited extent in the northwestern part of the Highland Rim, consists of poorly sorted ooliths developed around the fossil fragments in a matrix of clear crystalline calcite.