Marcellus Formation
Stratigraphic range: Middle Devonian
Marcellus shale exposure above Marcellus, N.Y. The vertical joints create sheer cliff faces.
Type	Geological formation
Unit of	Hamilton Group
Sub-units	See: Named members
Underlies	Mahantango Formation and Skaneatales Formation/Stafford Lime Mbr
Overlies	Huntersville Chert, Needmore Shale, and Onondaga Formation
Thickness	up to 900 feet (270 m)[1]
Lithology
Primary	Shale
Other	Slate, limestone, sandstone, tuff
Location
Region	Appalachian Basin of eastern North America
Extent	600 miles (970 km)[2]
Type section
Named for	Marcellus, New York
Named by	James Hall, 1839

The Marcellus Formation or the Marcellus Shale is a Middle Devonian age unit of sedimentary rock found in eastern North America. Named for a distinctive outcrop near the village of Marcellus, New York,^[3] it extends throughout much of the Appalachian Basin.^[4][5][6]

The unit name usage by the U.S. Geological Survey (USGS) includes Marcellus Shale and Marcellus Formation.^[7] The term "Marcellus Shale" is the preferred name throughout most of the Appalachian region, although the term "Marcellus Formation" is also acceptable within the State of Pennsylvania.^[7] The unit was first described and named as the "Marcellus shales" by J. Hall in 1839.^[8]