| Reedsville Formation | |
| Type: | sedimentary |
| Age: | Late Ordovician |
| Period: | Late Ordovician |
| Prilithology: | shale |
| Otherlithology: | sandstone |
| Namedfor: | Reedsville, Pennsylvania |
| Namedby: | E. O. Ulrich[1] |
| Region: | Appalachian Mountains |
| Unitof: | Chickamauga Group (TN only)[2] |
| Underlies: | Bald Eagle Formation and Juniata Formation |
| Overlies: | Antes Gap Shale, Trenton Limestone in TN and Utica Shale in WV |
| Extent: | Maryland, Pennsylvania, Tennessee, Virginia,[3] West Virginia |
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.[4] 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.[5]
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.[6]
Isotopic dating of shale mylonite in Pennsylvania reveals a K-Ar age of 372+/-8 Ma.[7]
The Reedsville is quarried locally in borrow pits for road material and fill.