A cross section of the strata between the top of the Oriskany Sandstone and the base of the Brallier Formation is based on 13 wells and 5 outcrops and extends for 150 mi (242 km) at lat. 39°43^primeN from the Ohio River eastward along the south border of Pennsylvania to the most eastern Appalachian outcrop belt. The two best time markers in the section are the Tioga Bentonite and Tully Limestone; the intervening strata thicken from a minimum of 490 ft (149 m) at the west end to a maximum of 1,850 ft (564 m) near the east end. Thickening is caused by stratigraphic divergence accompanying eastward increase in grain size of the clastic deposits which entered the Appalachian basin from the east. The stratigraphic section illustrates facies changes and thickness relations of these formations and their members: Needmore Shale, Huntersville Chert, Marcellus Shale, Mahantango Formation, Harrell Shale, Millboro Shale, and Brallier Formation.