A thin sequence of Silurian and Devonian carbonate strata (maximum thickness at the surface about 450 feet) crops out in the Arbuckle Mountains region, the Criner Hills, and in northeastern Oklahoma. In the Arbuckle Mountains region these strata are referred to six discrete depositional units which comprise the Hunton Group, the oldest being the Keel and Ideal Quarry Members of the Chimneyhill Formation of Early Silurian age (Alexandrian), and the youngest the Frisco Formation of Early Devonian age (Deerparkian); in northeastern Oklahoma (Sequoyah County) the Frisco Formation is locally overlain by the Sallisaw Formation of latest Early Devonian age (Esopusian). These depositional units are believed to be separated from one another by periods of time during which there wa induration and, in places, uplift and erosion. An unconformity separates the Silurian from the Devonian strata, and throughout much of the southeastern part of the Arbuckle Mountains region Early Devonian rocks (Helderbergian) rest on Early Silurian rocks (Alexandrian) and are within a few feet of the Ordovician strata. In Sequoyah County very latest Early Devonian strata (Esopusian) locally rest on late Niagaran strata.