The Yorktown Formation is a mapped bedrock unit in the

Coastal Plain A coastal plain (also coastal plains, coastal lowland, coastal lowlands) is an area of flat, low-lying land adjacent to a sea coast. A fall line commonly marks the border between a coastal plain and an upland area. Formation Coastal plains can f ...

Maryland Maryland ( ) is a U.S. state, state in the Mid-Atlantic (United States), Mid-Atlantic region of the United States. It borders the states of Virginia to its south, West Virginia to its west, Pennsylvania to its north, and Delaware to its east ...

Virginia Virginia, officially the Commonwealth of Virginia, is a U.S. state, state in the Southeastern United States, Southeastern and Mid-Atlantic (United States), Mid-Atlantic regions of the United States between the East Coast of the United States ...

North Carolina North Carolina ( ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, South Carolina to the south, Georgia (U.S. stat ...

and

South Carolina South Carolina ( ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders North Carolina to the north and northeast, the Atlantic Ocean to the southeast, and Georgia (U.S. state), Georg ...

. It is overconsolidated and highly

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

iferous.

Description

The Yorktown is composed largely of overconsolidated

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

and

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

with abundant calcareous shells, primarily

bivalves Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed by a calcified exoskeleton consis ...

Stratigraphy

The Yorktown

unconformably An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

overlies the Miocene Eastover Formation, and conformably underlies the Pliocene Croatan Formation. The Yorktown was divided into members by Ward and Blackwelder (1980). These are in ascending order: Sunken Meadow Member, Rushmere Member, Morgarts Beach Member, and Moore House Member. The uppermost Tunnels Mill Member is recognized in Maryland only.

Notable exposures

*Type Section:

Yorktown, Virginia Yorktown is a town in York County, Virginia, United States. It is the county seat of York County, one of the eight original shires formed in Colony of Virginia, colonial Virginia in 1682. Yorktown's population was 195 as of the 2010 census, while ...

on southwest side of York River, York County *Carters Grove Bluffs, north side of

James River The James River is a river in Virginia that begins in the Appalachian Mountains and flows from the confluence of the Cowpasture and Jackson Rivers in Botetourt County U.S. Geological Survey. National Hydrography Dataset high-resolution flowli ...

*Moore House Bluff, southwest side of York River, York County (very close to type section)

Age

Hazel (1971) revised the age of the Yorktown from Miocene to Late

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

to Early

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58ostracod Ostracods, or ostracodes, are a Class (biology), class of the crustacean, Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 33,000 species (only 13,000 of which are extant taxon, extant) have been identified,Brandão, S.N.; Antoni ...

biostratigraphy Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. "Biostratigraphy." ''Oxford Reference: Dictionary of Biology ...

. The age was revised by Gibson (1983) to extend into the Middle Pliocene based on

foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...

. Further biostratigraphic work with ostracods and foraminifera was completed by Cronin (1991), which also summarized previous investigations.

Fossils

Bivalves Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed by a calcified exoskeleton consis ...

, including '' Glycymeris subovata'', ''

Chesapecten jeffersonius '' Chesapecten jeffersonius '' is the fossilized form of an extinct scallop, which lived in the early Pliocene epoch between four and five million years ago on Virginia's coastal plain. ''Chesapecten jeffersonius'' are commonly found in strata ...

'', ''Chesapecten madisonius'', '' Mercenaria tridacnoides'', ''Panopera reflexa'', '' Chama'', ''

Ensis ''Ensis'' is a genus of medium-sized edible saltwater clams, littoral bivalve molluscs in the family Pharidae. ''Ensis'', or razor clams, are known in much of Scotland as spoots, for the spouts of water they eject while burrowing into the san ...

'', ''Striarca'' and ''Noetia'' (see Noetiidae), '' Cerastoderma'', '' Dosinia'', '' Mulinia'', ''

Kuphus ''Kuphus'' is a genus of shipworms, marine Bivalvia, bivalve Mollusca, molluscs in the family (biology), family Teredinidae. While there are four Extinction, extinct species in the genus,Maempel, George Zammit. "Kuphus melitensis, a new teredinid ...

'' (Shipworm), ''

Panope In Greek mythology, Panopea (Ancient Greek: Πανόπεια ''Panopeia'') or Panope (Πανόπη) may refer to various characters. The names mean 'panorama' or means 'of the beautiful husband'. * Panope or Poenope,Hyginus, ''Fabulae'' Preface ...

'' (Geoduck), and the oyster ''

Ostrea ''Ostrea'' is a genus of edible oysters, marine bivalve mollusks in the family Ostreidae, the oysters. Fossil records Although molecular studies suggest that ''Ostrea'' first appeared around the Eocene and originated no earlier than the Cre ...

'' *

Gastropods Gastropods (), commonly known as slugs and snails, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and from the land. Ther ...

, including '' Crucibulum'', ''

Calliostoma ''Calliostoma'' is a genus of small to medium-sized sea snails with gills and an operculum, marine gastropod molluscs within the family Calliostomatidae, the Calliostoma top snails. Previously this genus was placed within the family Trochid ...

'', ''

Busycon ''Busycon'' is a genus of very large edible sea snails in the subfamily Busyconinae. These snails are commonly known in the United States as ''whelks'' or ''Busycon whelks''. Less commonly they are loosely, and somewhat misleadingly, called "conc ...

'', ''

Turritella ''Turritella'' is a genus of medium-sized sea snails with an operculum, marine gastropod mollusks in the family Turritellidae.Vos, C.; Gofas, S. (2013). Turritella Lamarck, 1799. Accessed through: World Register of Marine Species at http://www. ...

'', and ''

Crepidula ''Crepidula'', commonly known as the slipper snails, slipper limpets, or slipper shells, is a genus of sea snails, marine (ocean), marine gastropod mollusks in the family Calyptraeidae. This family includes the slipper snails (''Crepidula''), hat ...

'' *

Foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...

, including the biostratigraphic marker species ''Dentoglobigerina altispira'' (see

Globigerinida The Globigerinina is a suborder of foraminiferans that are found as marine plankton. They produce hyaline calcareous tests, and are known as fossils from the Jurassic period onwards. The group has included more than 100 genera and over 400 spec ...

), ''Sphaeroidinellopsis'', and ''Globorotalia puncticulata''Dowsett, H.J., and Wiggs, L.B., 1992, Planktonic foraminiferal assemblage of the Yorktown Formation, Virginia, USA: Micropaleontology, v. 38, no. 1, p. 75-86. * Scleractinian corals, including '' Septastrea marylandica'', '' Paracyathus vaughani'' (see

Caryophylliidae The Caryophylliidae are a family (biology), family of stony corals found from the tropics to temperate seas, and from shallow to very deep water. Genera *''Africana (coral), Africana'' Ocana & Brito, 2015 *''Anomocora'' Studer, 1878 *''Aste ...

), and ''Astrangia lineata'' *

Ostracod Ostracods, or ostracodes, are a Class (biology), class of the crustacean, Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 33,000 species (only 13,000 of which are extant taxon, extant) have been identified,Brandão, S.N.; Antoni ...

s *

Bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic animal, aquatic invertebrate animals, nearly all living in sedentary Colony (biology), colonies. Typically about long, they have a spe ...

ns *

Barnacle Barnacles are arthropods of the subclass (taxonomy), subclass Cirripedia in the subphylum Crustacean, Crustacea. They are related to crabs and lobsters, with similar Nauplius (larva), nauplius larvae. Barnacles are exclusively marine invertebra ...

s, including ''

Balanus ''Balanus'' is a genus of barnacles in the family Balanidae of the subphylum Crustacea. This genus is known in the fossil record from the Jurassic to the Quaternary periods (age range: from 189.6 to 0.0 million years ago.). Fossil shells withi ...

'' *

Worm Worms are many different distantly related bilateria, bilateral animals that typically have a long cylindrical tube-like body, no limb (anatomy), limbs, and usually no eyes. Worms vary in size from microscopic to over in length for marine ...

s *

Sponge Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and a ...

s *

Bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s, including the large pelican '' Pelecanus schreiberi''. *

Whales Whales are a widely distributed and diverse group of fully Aquatic animal, aquatic placental mammal, placental marine mammals. As an informal and Colloquialism, colloquial grouping, they correspond to large members of the infraorder Cetacea ...

, including the prehistoric sperm whale ''

Scaldicetus ''Scaldicetus'' is an extinct genus of highly predatory macroraptorial sperm whale. Although widely used for a number of extinct physeterids with primitive dental morphology consisting of enameled teeth, ''Scaldicetus'' as generally recognized ...

''.

Gallery

Image:coral_bivalve_Yorktown_Fm.jpg,

Coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...

growing on a ''Crepidula'' sp. (slipper snail) from the Yorktown Formation Image:Rushmere_Mbr_Yorktown_Fm.jpg, Rushmere Member of the Yorktown Formation, with abundant ''Chama''

Image:Sunken_Meadow_Mbr_Yorktown_Fm.jpg, Sunken Meadow Member of the Yorktown Formation Image:Balanus_Yorktown_Fm.jpg, ''

sp.'' (barnacles) from Carters Grove Bluffs outcrop, growing on a bivalve

References

{{reflist Neogene geology of Virginia Neogene geology of North Carolina