Hamilton Group

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This article is about the geological structure. For other uses, see Hamilton Group (disambiguation).
Hamilton Group or
Millboro Shale
Stratigraphic range: Middle Devonian
Type Geological unit
Sub-units Mahantango Formation
Marcellus Formation
Thickness up to 2,500 feet (800 m)
Primary Shale
Other Limestone, Siltstone, Sandstone, Claystone
Region Appalachian Basin of
eastern North America
Extent New York, Pennsylvania, Maryland, Virginia
Type section
Named for Hamilton, New York
Named by James Hall

The Devonian Hamilton Group is a mapped bedrock unit in New York, Pennsylvania, Maryland, Ohio and West Virginia. In Virginia, it is known as the laterally equivalent Millboro Shale.[1] The group is named for the village of Hamilton, New York.[2] These rocks are the oldest strata of the Devonian gas shale sequence.[3]

Generalized stratigraphic nomenclature for the Middle Devonian strata in the Appalachian Basin.[4]


The Hamilton Group consists of the Mahantango Formation, a gray, dark gray, brown, and olive laminated shale; siltstone; and very fine-grained sandstone or claystone containing marine fossils. It overlies the Marcellus Shale, a fissile gray-black to black, thinly laminated, pyritic, carbonaceous thin shale with sparse marine fauna and siderite concretions. The total thickness of the Hamilton Group in Pennsylvania runs about 970 feet. In New York State, it thickens from 250 feet near Lake Erie to over 2,500 feet in Ulster and Greene counties. Depths ranging from outcrops to 8,000 feet below the surface of Sullivan County, in the southeastern part of the state.[3]

In the interior lowlands of New York, the Hamilton Group contains the Marcellus, Skaneateles, Ludlowville, and Moscow Formations, in ascending order, with the Tully Limestone above.[5] These units are divided by the Stafford, Centerfield, and Tichenor limestones.[6] In Ontario, Canada, the Hamilton Group formations are, in ascending order, Bell, Rockport Quarry, Arkona, Hungry Hollow, Widder, and Ipperwash; the Kettle Point Formation of the late Devonian lies unconformably above.[7]

The Mahantango Formation includes these members in descending order: Tully argillaceous limestone, Sherman Ridge, Montebello sandstone, Fisher Ridge, Dalmatia, and Turkey Ridge. In south-central Pennsylvania, it includes Clearville, Frame, Chaneysville, and Gander Run Members. Its thickness in Maryland ranges from 600 feet in the west, increasing to 1,200 feet in the east,[8] and approximately 1000 feet thick in central Pennsylvania.[9]

The Marcellus Formation contains a local limestone Purcell Member and Tioga bentonite at the base in eastern Pennsylvania. Its thickness in Maryland ranges from 250 feet in east, increasing to 500 feet in the west,[8] but is only about 70 feet thick in central Pennsylvania.[9]

The Hamilton Group, Tioga Metabentonite Bed, and Needmore Shale were formerly called the Romney Formation.[8]


  • The Geneseo Shale is a dark grey to black shale that overlies the Tully Limestone; it includes the Filmore beds.[10]
  • The Tully Limestone is a shallow-water carbonaceous unit.
  • The Moscow Formation (Middle Devonian / Givetian) comprises grey to black shales interbedded with limestones and calcareous mudstones and usually bioturbated.[11]


The uppermost clay regions of the Mahantango Formation are rich in fossils.[9]


The distribution of the unit can be observed on this map of Hudson-Mohawk bedrock scans.


Relative age dating of the Hamilton places it in the middle and lower Devonian period. In Maryland, it rests conformably atop the Tioga Metabentonite Bed of thinly laminated shale with a thickness less than one foot over the Needmore Shale, and conformably below the Brallier Formation (formerly Woodmont Shale) dark laminated shale in the east, Harrell Shale dark laminated shale in the west, and Tully Limestone in the subsurface of Garrett County.[8] In Pennsylvania it lies atop the Onondaga Formation limestone and shale forming a sharp conformable contact with the Selinsgrove Limestone of that formation, and lies beneath the Portage (Brallier, Harrell, Burket) dark shales of the Harrell Formation.[12]

Economic uses[edit]

The Hamilton is a good source of road material, riprap and building stone,[13] that is used locally for shale aggregate and common fill.[9]


  1. ^ "Map Unit Descriptions" (pdf). Virginia Department of Mines, Minerals and Energy. 2007-09-28. 
  2. ^ Wood, G.H., Trexler, J.P., Kehn, T.M., (1964). Geology of the West-Central Part of the Southern Anthracite Field and Adjoining Areas, Pennsylvania. United States Geological Survey, C-46.
  3. ^ a b Martin, John P. "The Middle Devonian Hamilton Group Shales in the Northern Appalachian Basin: Production and Potential". New York State Energy Research and Development Authority. Archived from the original on 2006-10-11. Retrieved 2008-04-02. 
  4. ^ Milici, Robert C.; Swezey, Christopher S. (2006). "Assessment of Appalachian Basin Oil and Gas Resources: Devonian Shale–Middle and Upper Paleozoic Total Petroleum System" (PDF). Open-File Report Series 2006-1237. United States Geological Survey. Retrieved 2008-04-05. 
  5. ^ Sinclair, James P. (1993). Final Generic Environmental Impact Statement for Promulgation of 6 NYCRR Part 382: Regulations for Low-Level Radioactive Waste Disposal Facilities. DIANE Publishing. ISBN 1-56806-746-1. 
  6. ^ Goldman, D.; Mitchell, C.E. (1990). "Morphology, systematics, and evolution of Middle Devonian Ambocoeliidae (Brachiopoda), western New York". Journal of Paleontology. 64 (1): 79–99. ISSN 0022-3360. JSTOR 1305546. 
  7. ^ "Geology of the Southwestern District". Resident Geologist Program - Southern Ontario. Ontario Ministry of Northern Development and Mines. 
  8. ^ a b c d "Geologic Maps of Maryland:". Retrieved 2008-01-26. 
  9. ^ a b c d McElroy, Thomas A.; Hoskins, Donald M. (2007). "Bedrock Geology of the Allensville Quadrangle, Huntingdon and Mifflin Counties, Pennsylvania" (pdf). Pennsylvania Geological Survey. 
  10. ^ Baird, Gordon C. (March 2001). Eustatic and Flexural events recorded in the Late Middle Devonian Tully Formation, New York State and Pennsylvania. Retrieved 2013-12-08. 
  11. ^ "Moscow Formation". United States Geological Survey. Retrieved 2011-08-30. 
  12. ^ Jackson, Margaret S.; Hanley, Peter M.; Sak, Peter B. (2007). "Preliminary Bedrock Geologic Map of the Middle Portion of the Susquehanna River Valley, Cumberland, Dauphin, And Perry Counties, Pennsylvania" (pdf). Open File Report OFBM-07-05.0. Pennsylvania Geological Survey. Retrieved 2008-01-26. 
  13. ^ McElroy, Thomas A.; Hoskins, Donald M. (2007). "Preliminary Bedrock Geologic Map of the Middle Portion of the Susquehanna River Valley, Cumberland, Dauphin, and Perry Counties, Pennsylvania" (pdf). Pennsylvania Geological Survey. 

See also[edit]

Coordinates: 40°44′05″N 77°46′42″W / 40.73472°N 77.77833°W / 40.73472; -77.77833