The Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 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

Slope-former

The term slope-former is a geologic term indicating that a unit of bedrock is less resistant to erosion than overlying or underlying units and consequently outcrops have low slope angles. It may be contrasted with cliff-former. Typical slope formers include shales, and limestones in humid...

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

Cliff-former

The term cliff-former is a geologic term indicating that a unit of bedrock is more resistant to erosion than overlying or underlying units and consequently outcrops have high slope angles. It is more or less equivalent to ridge-former, and may be contrasted with slope-former. In humid...

 Bald Eagle

Bald Eagle Formation

The Ordovician Bald Eagle Formation is a mapped bedrock unit in central Pennsylvania, USA. It is a ridge-forming unit in the Appalachian Mountains.-Description:...

 sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

 unit in the Appalachian Mountains

Appalachian Mountains

The Appalachian Mountains #Whether the stressed vowel is or ,#Whether the "ch" is pronounced as a fricative or an affricate , and#Whether the final vowel is the monophthong or the diphthong .), often called the Appalachians, are a system of mountains in eastern North America. The Appalachians...

. It is often abbreviated Or on geologic maps.

Description

The Reedsville Formation is an Olive-gray to dark-gray siltstone

Siltstone

Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...

, shale

Shale

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...

, and fine-grainded sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

. In Central Pennsylvania along the Nittany Arch

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...

, and extending into the subsurface of northern West Virginia

West Virginia

West Virginia is a state in the Appalachian and Southeastern regions of the United States, bordered by Virginia to the southeast, Kentucky to the southwest, Ohio to the northwest, Pennsylvania to the northeast and Maryland to the east...

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

Age

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

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 and conformably below the Bald Eagle Formation

Bald Eagle Formation

The Ordovician Bald Eagle Formation is a mapped bedrock unit in central Pennsylvania, USA. It is a ridge-forming unit in the Appalachian Mountains.-Description:...

.

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