Hamilton Group

Hamilton Group or
Millboro Shale
Hamilton Group or; Millboro Shale
Stratigraphic range: Eifelian - Famennian
Type	Group
Sub-units	Mahantango Formation; Clearville Siltstone Frame Shale Donation Siltstone Crooked Creek Shale Backbone Ridge Siltstone Chaneysville Siltstone Gander Run Shale Moscow Formation; North Evans limestone Windom Shale Kashong Shale Tichenor Limestone Ludlowville Formation; Jaycox Shale Owasco Shale Spafford Shale Wanakah Shale, Ivy Point Shale Ledyard Shale, Otisco Shale Centerfield Limestone Skaneatales Formation; Levanna Member Stafford Limestone Marcellus Formation; Otaka Creek Member Cherry Valley Member Union Spring Member Tioga Bentonites
Underlies	Tully Limestone and Harrell Shale
Overlies	Onondaga Limestone
Thickness	up to 2,500 feet (800 m)
Lithology
Primary	Shale
Other	Limestone, Siltstone, Claystone
Location
Region	Appalachian Basin of; eastern North America
Extent	Maryland, New York, Pennsylvania, Virginia, West Virginia and Ontario, Canada
Type section
Named for	Hamilton, New York
Named by	James Hall

The Hamilton Group is a Devonian-age geological group which is located in the Appalachian region of the United States. It is present in New York, Pennsylvania, Maryland, Ohio, West Virginia, northwestern Virginia and Ontario, Canada, and is mainly composed of marine shale with some sandstone.

There are two main formations encompassed by the group: the Mahantango Formation and the Marcellus Shale. In southwestern Virginia, where the two sub-units are not easily distinguishable, the Hamilton Group is broadly equivalent to the Millboro Shale or Millboro Formation.

The group is named for the village of Hamilton, New York. These rocks are the oldest strata of the Devonian gas shale sequence.

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