The Marcellus Formation, also classified as the Marcellus Subgroup of the Hamilton Group, Marcellus Member of the Romney Formation, or simply the Marcellus Shale, is a mapped bedrock unit in eastern North America. It is a sedimentary rock formed from marine deposits. Named for a distinctive outcrop near the village of Marcellus, New York in an 1839 geological survey, it extends throughout much of the Appalachian Basin. The energy development community is drilling deep beneath the Allegheny Plateau to reach its largely untapped natural gas reserves located near the high-demand markets along the East Coast of the United States.
Stratigraphically, the Marcellus is the lowest unit of the Devonian age Hamilton Group, and is divided into several named sub-units. Although black shale is the dominant lithology, it also contains lighter shales and interbedded limestone layers due to sea level variation during its deposition almost 400 million years ago. The black shale was deposited in relatively deep water devoid of oxygen, making it only sparsely fossiliferous. Most fossils are contained in the limestones and the fossil record in these layers provides important paleontologal insights on faunal turnovers. The black shales also contain iron ore, uranium and pyrite and the fissile shales are easily eroded, presenting civil and environmental engineering challenges.