High-grade limestone is present in several Middle Ordovician formations in Central Pennsylvania, but only the Valentine member of the Curtin formation has sufficient thickness for large-scale removal. The Valentine is restricted to outcrop belts northwest of a line from east of Lock Haven and Rebersburg to north of Millheim, Spring Mills, Lemont and Stormstown, in Clinton and Centre counties. The maximum thickness is 90 feet at Bellefonte, the upper 65 feet having less than two per cent of non-carbonate. Regional unconformity at the base of the overlying Nealmont limestone gradually reduces the unit toward the line of extinction, restricting the area having important thickness of stone. Moreover, the Valentine is considerably reduced in about 10 per cent of the area of outcrop in the Bellefonte district by channels filled with Nealmont limestone. Much of the stone has been removed from the most favorable areas of outcrop, but a large reserve is available to mining.