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Reedsville Formation
Stratigraphic range: Late Ordovician
Reedsville Formation 522.jpg
Outcrop of Reedsville Formation on south side of U.S. Route 522, Blacklog Gap, Huntingdon County, Pennsylvania.
Type sedimentary
Unit of Chickamauga Group (TN only)
Underlies Bald Eagle Formation and Juniata Formation
Overlies Antes Gap Shale, Trenton Limestone in TN and Utica Shale in WV
Primary shale
Other sandstone
Region Appalachian Mountains
Extent Maryland, Pennsylvania, Tennessee, Virginia, West Virginia
Type section
Named for Reedsville, Pennsylvania
Named by E. O. Ulrich

The Ordovician Reedsville Formation is a mapped surficial bedrock unit in Pennsylvania, Maryland, Virginia, West Virginia, and Tennessee, that extends into the subsurface of Ohio. This rock is a slope-former adjacent to (and stratigraphically below) the prominent ridge-forming Bald Eagle sandstone unit in the Appalachian Mountains. It is often abbreviated Or on geologic maps.


The Reedsville Formation is an olive-gray to dark-gray siltstone, shale, and fine-grained sandstone. In Central Pennsylvania along the Nittany Arch, and extending into the subsurface of northern West Virginia, the base of the Reedsville formation includes the black calcareous Antes Shale formation.

Reedsville Fm calc ss
Etched section of carbonaceous sandstone bed of Reedsville Formation from along Rt. 36 near Loysburg, Bedford Co., PA. Soft-sediment deformation evident. Contains sparse fossils (black in section image) - probably brachiopods and bryozoans.

Type section

The type locality is at Reedsville, Pennsylvania.


Relative age dating of the Reedsville places it in the Upper Ordovician. It rests conformably atop the Upper Ordovician Coburn Formation at the top of the Trenton Group limestone and conformably below the Bald Eagle Formation.

Isotopic dating of shale mylonite in Pennsylvania reveals a K-Ar age of 372+/-8 Ma.

Economic uses

The Reedsville is quarried locally in borrow pits for road material and fill.


Taxa Class Description Images
  • Kinoceras
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