Correlation of post-Tully, Upper Devonian strata is based on the combination of lithology, faunal assemblages, and thicknesses. Black shales are laterally persistent key beds amid interfingering gray shales, siltstones, mudstones, and sandstones. The rate of maximum thickening was found to increase exponentially S. 60 degrees E., using the top of the Tully Limestone as a key horizon. Five formations are recognized in the W.-central area (Watkins Glen-Elmira) and were traced into the eastern part (Ithaca-Owego). At the base is the Geneseo Formation, chiefly black shale, overlain by the Ithaca Formation. The Ithaca is divided into, in ascending order, the Penn Yan, Genundewa, and West River members in the W., and the Sherburne, Renwick, and Treman members in the E. The overlying Middlesex Formation, composed essentially of black shale, consists of the basal Montour Member, the Johns Creek Member, and the upper Sawmill Creek Member. Next above is the Sonyea Formation, represented by the redefined Cashaqua Member in western New York, by the Pulteney, Rock Stream, and Rye Point members (in ascending order) in W.-central New York, and the West Danby Member near Ithaca. The upper formation, the Rhinestreet, consists of, in ascending order, the Moreland, Millport, Dunn Hill, Beers Hill, and Roricks Glen members in the W. To the E., only the Moreland Member is distinguishable, and the upper members are considered part of the Chemung Formation.