ABSTRACT: Pennsylvanian rocks are at the bedrock surface in the Illinois Basin, which covers most of central and southern Illinois, southwestern Indiana, and part of western Kentucky. The lower contact is a regional unconformity, incised into rocks ranging in age from Ordovician to Late Mississippian. Morrowan rocks are confined to the southeastern part of the basin, the bottoms of deep paleovalleys, and a small outlier at the northwest corner. Characteristic rocks are clean quartz sandstone (quartz arenite), siltstone, shale, lenticular coal, and rare limestone. Atokan rocks show a transition from quartz arenite to lithic arenite, and progressively wider depositional extent as valleys on the sub-Pennsylvanian surface were filled. Cyclic sedimentation became prominent during the Desmoinesian, with regionally extensive mineable coal seams and layers of marine limestone and black phosphatic shale. Similar patterns prevailed through Missourian and Virgilian time, except that coal beds are generally thinner and limestone beds thicker. Lower Permian rocks are confined to a small down-faulted outlier in Kentucky.