# Marcellus Shale Canonical page: https://lithomap.app/resources/geologic-formations/united-states/marcellus-shale Last reviewed: September 17, 2026 Page purpose: Comprehensive United States geologic formation reference for human readers, search systems, and language models. Major region: Appalachians Regional setting: Appalachian Basin Age: Middle Devonian Age detail: Early Middle Devonian, Eifelian; roughly 393–388 million years ago Depositional setting: Oxygen-poor marine basin and adjacent shelf Rock types: Black shale, Gray shale, Siltstone, Limestone States in recognized usage: Kentucky, Maryland, New Jersey, New York, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia ## Summary The Marcellus is an organic-rich Devonian marine shale of the Appalachian Basin, important both as a record of an ancient sea and as a deeply buried natural-gas reservoir whose name and rank vary across state lines. ## Distribution The Marcellus extends through much of the Appalachian Basin from New York and Pennsylvania into adjoining Mid-Atlantic and central Appalachian states. It is exposed in some folded and uplifted areas but lies deeply buried across much of its gas-producing extent. Formal usage changes geographically: maps may call it a shale, a formation, or a member within another formation or group. These are not necessarily contradictions; stratigraphic rank and bounding units can change as the rock body changes across the basin. ## Geological history Dark, fine-grained sediment accumulated in a marine basin where limited oxygen at the seafloor slowed the decay of organic matter. Periodic changes in circulation, sediment supply, and water depth produced lighter shale, siltstone, carbonate beds, and fossil-rich horizons. Burial converted organic matter into hydrocarbons and compacted the mud into low-permeability shale. Modern production generally depends on horizontal drilling and hydraulic fracturing, so surface outcrops do not resemble the engineered reservoir conditions found kilometers underground. ## Fossil record Fossils include marine brachiopods, bivalves, cephalopods, corals, crinoid debris, and microscopic organisms, with abundance controlled by oxygen levels and local facies. Some carbonate-rich or marginal intervals are much more fossiliferous than the darkest shale. A black, organic-rich layer may preserve few obvious body fossils even while retaining chemical and microscopic evidence of marine life. Fossil content should therefore be described by member and locality rather than assumed from the formation name alone. ## Minerals and geological resources Clay minerals, quartz, and carbonate minerals such as calcite and dolomite make up much of the rock. Pyrite is common in oxygen-poor marine shale, and small amounts of feldspar and other detrital minerals occur. Natural gas is a geological resource derived from organic matter, not a mineral. Pyrite-bearing shale can generate acidic drainage when freshly exposed, and drilling-related minerals or salts at industrial sites should never be treated as collectible material. ## Field context - Much of the formation is subsurface; a production map is not the same thing as an outcrop map. - Roadcuts and quarries require owner or agency permission and may have falling-rock and traffic hazards. - Stratigraphic rank and member names differ among states, so cite the local geological survey map. ## Connected field guides - Shale (rock; Named rock type and mineral context) — https://lithomap.app/resources/rock-identification/shale - Limestone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/limestone - Siltstone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/siltstone - Calcite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/calcite - Dolomite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/dolomite - Feldspar (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/feldspar Mapped location and formation context support an identification but cannot establish one. Compare several physical properties and use laboratory analysis when the material remains ambiguous. ## Connected state geology guides - Pennsylvania geology — https://lithomap.app/resources/geology/united-states/pennsylvania ## Authoritative sources - Geolex Marcellus unit record — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/Units/Marcellus_2635.html - Appalachian Basin black-shale stratigraphic framework — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/resources/docs/USGS_B-1605-A_t.pdf ## Formation photograph - Image: https://lithomap.app/_delivery/formations/marcellus-shale.webp - Description: Dark Marcellus Shale exposed along a road near Marcellus, New York - Creator: Lvklock - License: CC BY-SA 3.0 — https://creativecommons.org/licenses/by-sa/3.0/ - Source record: https://commons.wikimedia.org/wiki/File:MarcellusShaleBank1.jpg Formation-level information does not prove that a fossil or mineral occurs at a specific coordinate. It also does not establish ownership, access, collecting permission, present conditions, or specimen identity.