More than five million tons of high-alumina clays occur in the Curwensville and Morgan Run districts of Clearfield County, Pennsylvania. These clays, associated chiefly with the Mercer member of the Pottsville group (Pennsylvanian), are flint, and diaspore-bearing flint clays. All gradations between flint clay and nearly pure diaspore occur at the Mercer horizon.The stratigraphic position of the nodule (diaspore-bearing) clays is now shown to be only at the base of the Mercer. Consequently a datum plane now exists in the economically important Pottsville rocks where formerly structural mapping depended on the Homewood sandstone, commonly lacking, or the overlying Lower Kittanning coal of the Allegheny group.The northeast-southwest trending Laurel Hill and Chestnut Ridge anticlines are the major structural features of the two districts. Normal faulting occurs in the Morgan Run district.The clay occurs in lenticular masses which have wide ranges of thickness. Both the thickness and sporadic distribution of the clay bodies are apparently influenced by the underlying eroded surface of the Connoquenessing sandstone.From the field relations and the petrographic textures and structures it is shown that the diaspore-clays were formed partly by the leaching of silica from flint clay and partly by addition of alumina to the flint clay.