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Wills Creek Formation is a mapped
Silurian The Silurian ( ) is a geologic period and system spanning 24.6 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 shortest period of the Paleozo ...
bedrock unit in
Pennsylvania Pennsylvania (; (Pennsylvania Dutch: )), officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, Mary ...
,
Maryland Maryland ( ) is a state in the Mid-Atlantic region of the United States. It shares borders with Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware and the Atlantic Ocean to it ...
,
Virginia Virginia, officially the Commonwealth of Virginia, is a state in the Mid-Atlantic and Southeastern regions of the United States, between the Atlantic Coast and the Appalachian Mountains. The geography and climate of the Commonwealth are ...
, and
West Virginia West Virginia is a state in the Appalachian, Mid-Atlantic and Southeastern regions of the United States.The Census Bureau and the Association of American Geographers classify West Virginia as part of the Southern United States while the ...
.


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 sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...
,
calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an adje ...
shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especially ...
,
dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...
,
argillaceous Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay minerals ...
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms when ...
, and
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...
. 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 Lewistown is a borough in and the county seat of Mifflin County, Pennsylvania, United States. It is the principal city of the '' Lewistown, PA Micropolitan Statistical Area'', which encompasses all of Mifflin County. It lies along the Juniata Ri ...
.


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 ...
, 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 a historic county in the far North West England. It covers part of the Lake District as well as the north Pennines and Solway Firth coast. Cumberland had an administrative function from the 12th century until 1974. From 1974 ...
,
Allegany County, Maryland Allegany County is located in the northwestern part of the U.S. state of Maryland. As of the 2020 census, the population was 68,106. Its county seat is Cumberland. The name ''Allegany'' may come from a local Lenape word, ''welhik hane'' or ''o ...
* Roundtop Hill, Maryland, along railroad grade


Age

Relative age dating of the Wills Creek places it in the
Silurian The Silurian ( ) is a geologic period and system spanning 24.6 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 shortest period of the Paleozo ...
period. It rests conformably a top the
Bloomsburg Formation The Silurian Bloomsburg Formation is a mapped bedrock unit in Pennsylvania, New Jersey, New York and Maryland. It is named for the town of Bloomsburg, Pennsylvania in which it was first described. The Bloomsburg marked the first occurrence of red s ...
and below the Tonoloway Formation.


Economic use

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


See also

*
Geology of Pennsylvania The Geology of Pennsylvania consists of six distinct physiographic provinces, three of which are subdivided into different sections. Each province has its own economic advantages and geologic hazards and plays an important role in shaping every ...


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 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