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 ...

Shawangunk Formation is a mapped bedrock unit in eastern

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, ...

New Jersey New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delaware ...

, and

New York New York most commonly refers to: * New York City, the most populous city in the United States, located in the state of New York * New York (state), a state in the northeastern United States New York may also refer to: Film and television * '' ...

. It is named for the

Shawangunk Ridge The Shawangunk Ridge , also known as the Shawangunk Mountains or The Gunks, is a ridge of bedrock in Ulster County, Sullivan County and Orange County in the state of New York, extending from the northernmost point of the border with New Jersey ...

for which it is the dominant rock type. The division of the Shawangunk between the

Tuscarora Formation The Silurian Tuscarora Formation — also known as Tuscarora Sandstone or Tuscarora Quartzite — is a mapped bedrock unit in Pennsylvania, Maryland, West Virginia, and Virginia, USA. Description The Tuscarora is a thin- to thick-bedded fine-gr ...

and Clinton Group has not been conclusively determined. The shift of nomenclature currently has the divide between

Hawk Mountain Hawk Mountain is a mountain ridge, part of the Blue Mountain Ridge in the Appalachian Mountain chain, located in central-eastern Pennsylvania near Reading and Allentown. The area includes of protected private and public land, including the H ...

and

Lehigh Gap The Lehigh Gap or Lehigh Water Gap is a water gap located in the townships of Lehigh, Washington, Lower Towamensing and East Penn in the Lehigh Valley region of eastern Pennsylvania. It was formed by the Lehigh River where it cuts through the ...

Description

The Shawangunk is defined as a light to dark-gray, fine to very coarse grained

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) ...

and conglomerate, containing a few

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, especial ...

interbeds.Berg, T.M., Edmunds, W.E., Geyer, A.R. and others, compilers, (1980). Geologic Map of Pennsylvania: Pennsylvania Geologic Survey, Map 1, scale 1:250,000. There are also four

members Member may refer to: * Military jury, referred to as "Members" in military jargon * Element (mathematics), an object that belongs to a mathematical set * In object-oriented programming, a member of a class ** Field (computer science), entries in ...

of this formation: Tammany, Lizard Creek, Minsi, and Weiders. The Lizard Creek member is described at Lehigh Gap as having conglomerates, sandstone, calcareous sandstone, siltstone, and shale with a few minor red beds. The Minsi Member is described as a conglomeritic quartzite and the Weiders is described as both a conglomerate and quartzite.Epstein, J.B., Sevon, W.D., Glaeser, J.D. (1974). Geology and the Mineral Resources of the Lehighton and Palmerton Quardrangles, Carbon and Northampton Counties, Pennsylvania. The Pennsylvania Geologic Survey, Harrisburg, PA. A195c&d. The Tammany and Lizard Creek members are equivalent to the Clinton Formation and the Minsi and Weiders members are equivalent to the Tuscarora Formation in the central and western part of Pennsylvania. The Tammany Member was not described at

Depositional environment

The

depositional environment In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be ...

of the Shawangunk has always been interpreted as mostly terrestrial to a shallow marine deposit resulting in a

molasse __NOTOC__ The term "molasse" () refers to sandstones, shales and conglomerates that form as terrestrial or shallow marine deposits in front of rising mountain chains. The molasse deposits accumulate in a foreland basin, especially on top of flys ...

sequence produced by the

Taconic orogeny The Taconic orogeny was a mountain building period that ended 440 million years ago and affected most of modern-day New England. A great mountain chain formed from eastern Canada down through what is now the Piedmont of the East coast of the Unit ...

. The numerous coarse-grained beds, especially in the lower part, suggest a relatively close orogenic source with very high to high energy fluvial systems depositing the sediments. Shale interbeds may indicate a slowing or shifting of these ancient river systems.

Fossils

Very few fossils exist in the Shawangunk; however,

Eurypterid Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the Order (biology), order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 Myr, million yea ...

s can be found in the middle portions of the formation roughly 420–750 feet from the base in some locations in New York State. Linguloid

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...

s were identified in the Lizard Creek Member. The

ichnofossil A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, ...

'' Arthrophycus'' was described in the Minsi Member in the area of Lehigh Gap. These were most likely made by

polychaete worms Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are m ...

of the family ''

Terebellida Terebellida make up an order of the Polychaeta class, commonly referred to as "bristle worms". Together with the Sabellida, the Spionida and some enigmatic families of unclear taxonomic relationship (e.g. the Saccocirridae), they make up the ...

''.

Notable Exposures

Delaware Water Gap Delaware Water Gap is a water gap on the border of the U.S. states of New Jersey and Pennsylvania where the Delaware River cuts through a large ridge of the Appalachian Mountains. The gap makes up the southern portion of the Delaware Water Gap N ...

Lehigh Gap, Pennsylvania The Lehigh Gap or Lehigh Water Gap is a water gap located in the townships of Lehigh, Washington, Lower Towamensing and East Penn in the Lehigh Valley region of eastern Pennsylvania. It was formed by the Lehigh River where it cuts through the ...

High Point State Park High Point State Park is a state park straddling the border of Wantage Township and Montague Township in Sussex County, within the Skylands Region of northwestern New Jersey, United States, near the border with New York State and Pennsylvania. ...

, New Jersey

Age

Relative age dating of the Shawangunk places it in the Lower

period, being deposited between 440 and 417 (±10) million years ago. It rests

unconformably An unconformity is a buried erosional or non-depositional 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 ...

above the Martinsburg Formation and conformably below 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 re ...

.Berg, T.M., et al., (1983). Stratagraphic Correlation Chart of Pennsylvania: G75, Pennsylvania Geologic Survey, Harrisburg, Pennsylvania.

Economic uses

The hardness of the quartzite and conglomerate layers make the Shawangunk an excellent building stone. Some of the boulders that have accumulated as

colluvium Colluvium (also colluvial material or colluvial soil) is a general name for loose, unconsolidated sediments that have been deposited at the base of hillslopes by either rainwash, sheetwash, slow continuous downslope creep, or a variable combinatio ...

at the base of Blue Mountain are used for decorative stone, building stone, and aggregate.

References

{{Reflist Shawangunks Silurian System of North America Silurian geology of Pennsylvania Geologic formations of New Jersey Geologic formations of New York (state) Sandstone in the United States Silurian southern paleotemperate deposits