The Late 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...

 Tonoloway Formation is a mapped 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....

 bedrock

Bedrock

In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...

 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 basal 50 m consists of medium-dark-gray laminated to thin-bedded calcisiltite

Calcisiltite

Calcisiltite is a type of limestone that is composed predominately, more than 50 percent, of detrital silt-size carbonate grains. These grains consist either of the silt-size particles of ooids, fragments of fossil shells, fragments of older limestones and dolomites, intraclasts, pellets, other...

 with shale partings and interbeds. Overlying 5 m are light-yellowish-gray to olive-gray mudstone and shale. Above this interval are 75 m of laminated calcisiltite with interbeds of thick to very thick bedded calcisiltite. The remainder of the formation is cyclic, consisting of three or four resistant ledges of laminated limestone and shale. Uppermost 20 m contains a variety of limestones. Lower contact with the Wills Creek is probably conformable. Upper contact is conformable and undulatory, occurring at the base of the "calico" limestone of the Keyser Formation

Keyser Formation

The Late Silurian to Early Devonian Keyser Formation is a mapped limestone bedrock unit in Pennsylvania, Maryland, Virginia, and West Virginia.-Description:...

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Depositional Environment

The depostional environment of the Tonoloway is interpreted as shallow marine.

Notable Exposures

• Tonoloway Ridge
  Tonoloway Ridge
  Tonoloway Ridge is a stratigraphic ridge that runs southwest northeast through the U.S. states of Pennsylvania, Maryland, and West Virginia. It reaches its highest elevation above mean sea level of 1,292 feet at a knob on its southern end in Morgan County, West Virginia...
  
   (type section)
• along Little Juniata River
  Little Juniata River
  ]The Little Juniata River, sometimes called the "Little J," is a river in central Pennsylvania. It is a tributary of the Juniata River in the Susquehanna River watershed. It is formed at Altoona by the confluence of several short streams...
  
  , 2 km northeast of Bellwood, Pennsylvania
  Bellwood, Pennsylvania
  Bellwood is a borough in Blair County, Pennsylvania, United States. It is part of the Altoona, PA Metropolitan Statistical Area. The population was 2,016 at the 2000 census.-Geography:Bellwood is located at ....