The

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

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 A slope-former is a unit of bedrock which is less resistant to erosion than overlying or underlying units and consequently results in outcrops with low relative slope angles. It may be contrasted with cliff-former. Typical slope forming lithol ...

adjacent to (and stratigraphically below) the prominent ridge-forming

Bald Eagle The bald eagle (''Haliaeetus leucocephalus'') is a bird of prey found in North America. A sea eagle, it has two known subspecies and forms a species pair with the white-tailed eagle (''Haliaeetus albicilla''), which occupies the same niche ...

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

unit in the

Appalachian Mountains The Appalachian Mountains, often called the Appalachians, are a mountain range in eastern to northeastern North America. The term "Appalachian" refers to several different regions associated with the mountain range, and its surrounding terrain ...

. It is often abbreviated Or on geologic maps.

Description

The Reedsville Formation is an olive-gray to dark-gray

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

, and fine-grained

. In Central Pennsylvania along the

Nittany Arch The Nittany Arch or Nittany anticline is an anticline geologic formation in the western part of the Ridge-and-Valley physiographic province of the Appalachian Mountains of Central Pennsylvania, United States. The Nitany Arch is more than 100 mile ...

, and extending into the subsurface of northern

West Virginia West Virginia is a mountainous U.S. state, state in the Southern United States, Southern and Mid-Atlantic (United States), Mid-Atlantic regions of the United States.The United States Census Bureau, Census Bureau and the Association of American ...

, the base of the Reedsville formation includes the black calcareous Antes Shale formation.

Type section

The type locality is at

Reedsville, Pennsylvania Reedsville is a census-designated place (CDP) in the Kishacoquillas Valley of Mifflin County, Pennsylvania, Mifflin County, Pennsylvania, United States. The population was 641 at the 2010 census. Reedsville has a high Amish population. General i ...

Age

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 Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

and conformably below the Bald Eagle Formation. Isotopic dating of shale

mylonite Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a ...

in Pennsylvania reveals a K-Ar age of 372+/-8 Ma.Lapham, D.M., and Root, S.I., 1971, Summary of isotopic age determinations in Pennsylvania: Pennsylvania Geological Survey Information Circular, 4th series, no. 70, 29 p.

Economic uses

The Reedsville is quarried locally in

borrow pit Digging, also referred to as excavation, is the process of using some implement such as claws, hands, manual tools or heavy equipment, to remove material from a solid surface, usually soil, sand or rock on the surface of Earth. Digging is actual ...

s for road material and fill.

Description

Type section

Age

Economic uses

Palaeontology

References

See also