# Yorktown Formation Canonical page: https://lithomap.app/resources/geologic-formations/united-states/yorktown-formation Last reviewed: September 17, 2026 Page purpose: Comprehensive United States geologic formation reference for human readers, search systems, and language models. Major region: Gulf / Atlantic Regional setting: Central Atlantic Coastal Plain Age: Pliocene Age detail: Mostly early to late Pliocene, approximately 4.5–3 million years old in well-dated sections Depositional setting: Shallow Atlantic shelf, embayments, and nearshore channels Rock types: Shelly sand, Clayey sand, Silt, Clay, Shell hash States in recognized usage: Maryland, North Carolina, South Carolina, Virginia ## Summary The Yorktown Formation is a shell-rich Pliocene marine deposit of the Atlantic Coastal Plain, preserving a diverse record of warm shallow seas before modern Chesapeake and Carolina landscapes formed. ## Distribution Yorktown deposits extend from Virginia and southern Maryland into North Carolina and parts of South Carolina, exposed along rivers, bluffs, excavations, and coastal-plain mines. Younger sediment commonly conceals the formation between exposures. Members and local facies range from shelly glauconitic sand to silty clay and cross-bedded shell hash. Historical usage included beds now assigned to other formations, so modern map definitions matter. ## Geological history Sand, mud, and abundant shells accumulated on a shallow continental shelf and within coastal embayments. Sea-level shifts moved shorelines and created repeated erosional surfaces, shell beds, and fining-upward sequences. Glauconite and phosphate formed or were reworked in marine settings, while later weathering leached shells from some updip deposits. Regional uplift and falling sea level exposed the deposits to river erosion. ## Fossil record Bivalves, gastropods, barnacles, corals, bryozoans, ostracodes, foraminifera, fish, sharks, rays, and marine mammals are documented. Large scallops and other mollusks can dominate shell beds. Fossil assemblages record water depth and temperature and vary among members. Fragile shells may remain intact in clay but become broken hash in higher-energy sand. ## Minerals and geological resources Quartz sand and clay minerals dominate, with calcite in shells and cement, plus glauconite and phosphate grains in some intervals. Iron oxides color weathered exposures. Sand and shell material have local aggregate significance, while phosphate-rich beds are regionally important. Mine and spoil access is controlled by owners and operating rules. ## Field context - River bluffs and excavated faces can collapse even when the sediment looks compact. - Do not enter mines or collect from spoil without current permission. - Member names and age assignments differ among states and historical reports. ## Connected field guides - Calcite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/calcite - Quartz (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/quartz 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 - No directly overlapping state guide is currently published. ## Authoritative sources - Geolex Yorktown references — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/UnitRefs/YorktownRefs_4487.html - Central Atlantic Coastal Plain stratigraphy — U.S. Geological Survey: https://pubs.usgs.gov/bul/1567/report.pdf ## Formation photograph - Image: https://lithomap.app/_delivery/formations/yorktown-formation.webp - Description: Layered Yorktown Formation exposed at Carters Grove Bluffs in Virginia - Creator: James Stuby - License: Public domain — https://creativecommons.org/publicdomain/mark/1.0/ - Source record: https://commons.wikimedia.org/wiki/File:Carters_Grove_Bluffs_Yorktown_Fm.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.