The Marcellus Shale, also known as the Marcellus Formation, is a unit of marine sedimentary rock believed to have been formed millions of years ago under inland seas throughout much of the Appalachian basin. Mircoscopic organisms were incorporated as the materials were deposited which slowly decayed to form a mixture of gases, which are still bound within the rock formation. The shale is named for a distinctive outcrop near the village of Marcellus, New York. The area is estimated to cover approximately 54,000 square miles in New York, Ohio, Pennsylvania and West Virginia, the largest natural gas region in the country.

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