The Silurian

Silurian

The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 Wills Creek Formation is a mapped bedrock unit in Pennsylvania

Pennsylvania

The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...

, Maryland

Maryland

Maryland is a U.S. state located in the Mid Atlantic region of the United States, bordering Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware to its east...

, Virginia

Virginia

The Commonwealth of Virginia , is a U.S. state on the Atlantic Coast of the Southern United States. Virginia is nicknamed the "Old Dominion" and sometimes the "Mother of Presidents" after the eight U.S. presidents born there...

, and 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...

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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

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,...

, calcareous

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:...

 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...

, dolostone

Dolostone

Dolostone or dolomite rock is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite. In old U.S.G.S. publications it was referred to as magnesian limestone. Most dolostone formed as a magnesium replacement of limestone or lime mud prior to lithification. It is...

, argillaceous 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 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,...

. 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, Pennsylvania

Lewistown is a borough in and the county seat of Mifflin County, Pennsylvania, United States. It lies along the Juniata River, northwest of Harrisburg. The number of people living in the borough in 1900 was 4,451; in 1910, 8,166; and in 1940, 13,017. The population was 8,998 at the 2000 census,...

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Fossils

The Wills Creek Limestone contain fossils from the Pridoli

Prídolí

Přídolí is a small town in Český Krumlov District in the South Bohemian Region of the Czech Republic.The town covers an area of , and has a population of 583 ....

 to the Ludlow epoch, or 422.9 to 418.1 Ma.

Dean et al. (1985) describe the Wills Creek as sparsely fossiliferous.

Conodont

Conodont

Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. Knowledge about soft tissues remains relatively sparse to this day...

s have been identified in the Wills Creek in Virginia (Ozarkodina snajdri crispa Zone).

Notable Exposures

• Type section at Wills Creek
  Wills Creek (North Branch Potomac River)
  Wills Creek is a tributary of the North Branch Potomac River in Pennsylvania and Maryland in the United States.Wills Creek drops off the Allegheny Mountains of southeastern Somerset County, Pennsylvania, and enters the North Branch Potomac River at Cumberland, Maryland.-History:thumb|220px|Fort...
  
   at Cumberland
  Cumberland, Maryland
  Cumberland is a city in the far western, Appalachian portion of Maryland, United States. It is the county seat of Allegany County, and the primary city of the Cumberland, MD-WV Metropolitan Statistical Area. At the 2010 census, the city had a population of 20,859, and the metropolitan area had a...
  
  , Allegany County, Maryland
  Allegany County, Maryland
  Allegany County is a county located in the northwestern part of the US state of Maryland. It is part of the Cumberland, MD-WV Metropolitan Statistical Area. As of 2010, the population was 75,087. Its county seat is Cumberland...
  
  
• Roundtop Hill
  Roundtop Hill (Maryland)
  Roundtop Hill is a mountain in Washington County, Maryland, southwest of the town of Hancock. The elevation at the summit is .-Geography:Unlike surrounding mountains, Roundtop Hill is not part of an extended ridge system, and is only about in length. It is located just under from the crest of...
  
  , Maryland, along railroad grade

Age

Relative age dating of the Wills Creek places it in the Silurian

Silurian

The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 period. It rests conformably

Unconformity

An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...

 a top the Bloomsburg Formation

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 for which it was first described. The Bloomsburg marked the first occurrence of red sedimentary rocks in the Appalachian Basin...

 and below the Tonoloway Formation

Tonoloway Formation

The Late Silurian Tonoloway Formation is a mapped limestone bedrock unit in Pennsylvania, Maryland, Virginia and West Virginia.-Description:The basal 50 m consists of medium-dark-gray laminated to thin-bedded calcisiltite with shale partings and interbeds. Overlying 5 m are light-yellowish-gray to...

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Economic Uses

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