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The Marcellus Formation is a black shale that may contain limestone beds and concentrations of iron pyrite (FeS2) and siderite (FeCO3).[7] Its sedimentary structure, or bedding, is moderately well-developed. Like most shales, it tends to split easily along the bedding plane, a property known as fissility.[7] Lighter colored shales in the upper portion of the formation tend to split into small thin-edged fragments after exposure.[8] These fragments may have rust stains from exposure of pyrite to air, and tiny gypsum (CaSO4·2H2O) crystals from the reaction between pyrite and limestone particles.[8] Fresh exposures of the pyriteiferous shale may develop the secondary mineralization of orange limonite (FeO(OH)·nH2O), and the pale yellow efflorescence or bloom of sulfur, associated with acid rock drainage.[9]
The Marcellus is found throughout the Allegheny Plateau region of the northern Appalachian Basin of North America. In the United States, the Marcellus shale runs across the Southern Tier and Finger Lakes regions of New York, in northern and western Pennsylvania, eastern Ohio, through western Maryland, and throughout most of West Virginia extending across the state line into extreme western Virginia.[21] The Marcellus bedrock in eastern Pennsylvania extends across the Delaware River into extreme western New Jersey.[1] It also exists in the subsurface of a small portion of Kentucky and Tennessee.[22] Below Lake Erie, it can be found crossing the border into Canada, where it stretches between Port Stanley and Long Point to St. Thomas in southern Ontario.
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