Wills Creek Formation is a mapped

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

bedrock unit in

Pennsylvania Pennsylvania, officially the Commonwealth of Pennsylvania, is a U.S. state, state spanning the Mid-Atlantic (United States), Mid-Atlantic, Northeastern United States, Northeastern, Appalachian, and Great Lakes region, Great Lakes regions o ...

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 ...

, and

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 ...

Description

The Wills Creek is defined as a moderately well bedded greenish-gray shale containing local limestone and sandstone zones, or more specifically as an olive to yellowish-gray, thin-bedded

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 ...

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime (mineral), lime or being chalky. The term is used in a wide variety of Science, scientific disciplines. In zoology ''Calcare ...

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 ...

, dolomite, argillaceous

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

. Red shale and siltstone occur in the lower part of the formation. The formation has a thickness between 450 feet and 600 feet in Maryland and 445 to 620 feet in Pennsylvania. The Wills Creek forms the bedrock of the valley around and to the east of Lewistown, Pennsylvania.

Fossils

The Wills Creek Limestone contain fossils from the Pridoli to the

Ludlow epoch In the geological timescale, the Ludlow Epoch (from 427.4 ± 0.5 million years ago to 423.0 ± 2.3 million years ago) occurred during the Silurian Period, after the end of the Homerian Age. It is named for the town of Ludlow in Shropshire, Engl ...

, or 422.9 to 418.1 Ma. Dean et al. (1985) describe the Wills Creek as sparsely fossiliferous. Conodonts have been identified in the Wills Creek in Virginia ('' Ozarkodina snajdri crispa'' Zone).Harris, A.G., Stamm, N.R., Weary, D.J., Repetski, J.E., Stamm, R.G., and Parker, R.A., 1994, Conodont color alteration index (CAI) map and conodont-based age determinations for the Winchester 30' x 60' quadrangle and adjacent area, Virginia, West Virginia, and Maryland: U.S. Geological Survey Miscellaneous Field Studies Map, MF-2239, 40 p., 1 sheet, scale 1:100,00

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Notable outcrops

* Type section at Wills Creek (North Branch Potomac River), Wills Creek at

Cumberland Cumberland ( ) is an area of North West England which was historically a county. The county was bordered by Northumberland to the north-east, County Durham to the east, Westmorland to the south-east, Lancashire to the south, and the Scottish ...

Allegany County, Maryland Allegany County is a County (United States), county located in the northwestern part of the U.S. state of Maryland. As of the 2020 United States census, 2020 census, the population was 68,106. Its county seat is Cumberland, Maryland, Cumberland ...

* Roundtop Hill, Maryland, along railroad grade

Age

Relative age dating of the Wills Creek places it in the

period. It rests

conformably Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general, strata are primarily igneous ...

a top the Bloomsburg Formation and below the Tonoloway Formation.

Economic use

The Wills Creek is a poor source of construction material and is only suitable as common fill.

References

{{Stratigraphic column of West Virginia Geologic formations of Maryland Geologic formations of Pennsylvania Geologic formations of Virginia Geologic formations of West Virginia Silurian System of North America Mudstone formations of the United States Limestone formations of the United States Sandstone formations of the United States Shale formations of the United States Silurian Maryland Silurian geology of Pennsylvania Silurian geology of Virginia Silurian West Virginia Silurian southern paleotemperate deposits Dolomite formations Siltstone formations of the United States

Description

Fossils

Notable outcrops

Age

Economic use

See also

References