With the exception of small inlier areas along the axes of Chestnut Ridge and Laurel Ridge anticlines, the Upper Devonian rocks of southwestern Pennsylvania are concealed beneath a mantle of Mississippian, Pennsylvanian, and Permian sediments. Thus, the study of this very important group of rocks becomes a problem for the subsurface stratigrapher over a third of the surface area of Pennsylvania. Drilling, in recent years, along Chestnut Ridge and Laurel Ridge anticlines, and also in the broad plateau area between Laurel Ridge and the Allegheny Front, has yielded many excellent sections based on detailed examinations of drill cuttings. Some of these sections, in a generalized graphic form, together with additional ones located in the oil- and gas-producing area west of Che tnut Ridge, are assembled into cross sections which illustrate the facies variations in the Upper Devonian sediments of southwestern Pennsylvania. The Conewango (uppermost Upper Devonian) age of the Devonian rocks exposed in the inlier areas of Chestnut Ridge and Laurel Ridge anticlines is confirmed. With the exception of the Huntersville chert and Oriskany sandstone, which are Lower Devonian in age, the producing sands of southwestern Pennsylvania all appear to be younger than the highest sub-Catskill marine beds exposed along the Allegheny Front, which are generally considered as being late Chemung in age.

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