# Calvert Formation Canonical page: https://lithomap.app/resources/geologic-formations/united-states/calvert-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: Chesapeake Bay and central Atlantic Coastal Plain Age: Early to middle Miocene Age detail: Mostly early to middle Miocene; local basal beds may be older Depositional setting: Shallow to outer marine shelf along the Atlantic margin Rock types: Sand, Clay, Marl, Diatomite, Silt States in recognized usage: Delaware, Maryland, Virginia ## Summary The Calvert Formation is a fossil-rich Miocene marine unit exposed spectacularly along Chesapeake Bay, recording changing shelf depths, productive waters, and repeated sedimentary cycles. ## Distribution The formation is best known from Calvert Cliffs in Maryland and extends beneath parts of Delaware and Virginia. Stream valleys, coastal cliffs, wells, and construction cuts reveal sections, though many are inaccessible or unstable. Members include diatomaceous, sandy, clayey, and marly intervals. Equivalent units replace the Calvert name farther north or south, and member boundaries are often recognized by fossils as well as rock type. ## Geological history Marine transgressions deposited basal sand followed by clay, silt, marl, and diatom-rich sediment as water deepened. Repeated fining-upward cycles record shoreline movement and changing sediment supply. Diatoms flourished in productive water, while shells accumulated during favorable bottom conditions. Later cliff erosion continually exposes and reworks fossils onto beaches. ## Fossil record Mollusks, diatoms, foraminifera, radiolarians, sharks, rays, bony fish, sea turtles, birds, whales, dolphins, and other marine vertebrates occur. Preservation and abundance differ strongly among beds. A fossil found loose on a beach may have eroded from a cliff and traveled, so its exact bed is usually unknown. Scientific context is far better when a specimen’s layer is documented lawfully. ## Minerals and geological resources Quartz, clay minerals, calcite, and biogenic silica dominate, with glauconite, phosphate, pyrite, and iron oxides locally present. Diatomaceous material and marl have local resource interest, but the formation’s greatest public value is its scientific fossil record. Cliff material should never be excavated where prohibited. ## Field context - Calvert Cliffs are actively eroding and can collapse without warning; remain away from cliff bases. - Tides can trap visitors against bluffs, and collecting rules vary by park and shoreline owner. - Loose beach fossils have reduced stratigraphic context even when identification is secure. ## Connected field guides - Calcite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/calcite - Pyrite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/pyrite - 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 Calvert unit record — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/Units/Calvert_793.html - Geolex Calvert references — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/UnitRefs/CalvertRefs_793.html ## Formation photograph - Image: https://lithomap.app/_delivery/formations/calvert-formation.webp - Description: Sedimentary cliffs exposing the Calvert Formation at Calvert Cliffs State Park, Maryland - Creator: Famartin - License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ - Source record: https://commons.wikimedia.org/wiki/File:2016-07-20_10_17_03_Cliffs_to_the_south_of_Grays_Creek_in_Calvert_Cliffs_State_Park%2C_Calvert_County%2C_Maryland.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.