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. 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. These units are divided by the Stafford, Centerfield, and Tichenor limestones. 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. 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, and approximately 1000 feet thick in central Pennsylvania. 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, but is only about 70 feet thick in central Pennsylvania. The Hamilton Group, Tioga Metabentonite Bed, and Needmore Shale were formerly called the Romney Formation.
The Geneseo Shale is a dark grey to black shale that overlies the Tully Limestone; it includes the Filmore beds.
The Tully Limestone is a shallow-water carbonaceous unit.
The Moscow Formation comprises grey to black shales interbedded with limestones and calcareous mudstones and usually bioturbated.
Fossils
The uppermost clay regions of the Mahantango Formation are rich in fossils.
Age
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 dark laminated shale in the east, Harrell Shale dark laminated shale in the west, and Tully Limestone in the subsurface of Garrett County. 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 dark shales of the Harrell Formation. In 2012, Read and Erikson reported the group as dating from the Eifelian to Famennian in Virginia.
