The Hammer Creek Formation is a mapped

bedrock In geology, bedrock is solid Rock (geology), rock that lies under loose material (regolith) within the crust (geology), crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface mater ...

unit consisting primarily of conglomerate, coarse

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

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 ...

. The Hammer Creek Formation was originally mapped as part of the

Gettysburg Formation The Gettysburg Formation is a mapped bedrock unit consisting primarily of sandstones, conglomerates, and shales. The Gettysburg Formation was first described in the Gettysburg area of Adams County, Pennsylvania in 1929, and over the following dec ...

Adams County, Pennsylvania Adams County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 103,852. Its county seat is Gettysburg. The county was created on January 22, 1800, from part of York County, and was named for the secon ...

in 1929. J. D. Glaeser renamed part of the Gettysburg to the Hammer Creek in 1963, to "avoid extending either the Gettysburg Formation from the west or the Brunswick Formation from the east to include rocks typical of neither unit." A major groundwater resources study of the Hammer Creek Formation and other formations of the Newark Supergroup in Pennsylvania was published by Charles R. Wood in 1980.Wood, C. R., 1980, Groundwater resources of the Gettysburg and Hammer Creek Formations, southeastern Pennsylvania: Pennsylvania Geological Survey, 4th ser., Water Resource Report 49, 87 p.
web release
.

Depositional environment

The Hammer Creek Formation and other formations of the Newark Supergroup were deposited in the Newark Basin, just one of many Triassic

rift basins In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben wi ...

existing on the east coast of North and South America, which formed as

plate tectonics Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...

pulled apart Pangaea into the

continent A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continents. Ordered from largest in area to smallest, these seven ...

s we see today.

Stratigraphy

The Hammer Creek Formation is

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 igne ...

underlain by the New Oxford Formation, which is the basal unit of the Newark Supergroup in south-central Pennsylvania. The Hammer Creek is mapped from the southern borders of Dauphin and

Lebanon Lebanon ( , ar, لُبْنَان, translit=lubnān, ), officially the Republic of Lebanon () or the Lebanese Republic, is a country in Western Asia. It is located between Syria to the north and east and Israel to the south, while Cyprus li ...

Counties to the northeast to the Schuylkill River. A laterally equivalent rock unit called the

Brunswick Formation The Passaic Formation is a mapped bedrock unit in Pennsylvania, New Jersey, and New York. It was previously known as the Brunswick Formation since it was first described in the vicinity of New Brunswick, New Jersey. It is now named for the city of ...

is mapped on the east side of the river and into New Jersey.Van Houten, F.B., 1980, Late Triassic part of Newark Supergroup, Delaware River section, west-central New Jersey, IN Manspeizer, Warren, ed., Field studies of New Jersey geology and guide to field trips: New York State Geological Association Guidebook, 52nd Annual Meeting, Newark, NJ, no. 52, p. 264-275.

Notable exposures

The type section is along Hammer Creek, Richland quadrangle, Lebanon County, Pennsylvania.

Age

Relative age dating of the Hammer Creek Formation places it in the Late

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

period.

References

{{reflist Triassic geology of Pennsylvania Geologic formations of New Jersey Triassic System of North America