The Silurian Bloomsburg Formation is a mapped 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, ...

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 ...

, New York and

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 is named for the town of Bloomsburg, Pennsylvania in which it was first described. The Bloomsburg marked the first occurrence of red sedimentary rocks in the

Appalachian Basin The geology of the Appalachians dates back to more than 480 million years ago. A look at rocks exposed in today's Appalachian Mountains reveals elongate belts of folded and thrust faulted marine sedimentary rocks, volcanic rocks and slivers of ...

Description

The Bloomsburg is defined as a grayish-red and greenish-gray

claystone Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too sm ...

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 ...

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

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 ...

, and very fine to fine-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) ...

, arranged in fining-upward cycles. Although the Bloomsburg can sometimes be green to gray, its most dominant color is red and they are often called "The Bloomsburg Red Beds". This is significant in the

since it marks the first large scale terrestrial collection of sediments in the rock record. Most of the sediments that were deposited before this time were marine, or a small scale terrestrial zone. In New Jersey, the Bloomsburg Red Beds were previously called the "High Falls Shale" or "High Falls Formation".

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 Bloomsburg was mostly terrestrial or shallow marine deposits 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 red color is often evidence that the sediments were deposited in arid or

oxidizing Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

conditions. The upper parts of the Bloomsburg are thought to be a marine transition zone.Hoskins, D.M., (1961). Stratigraphy and paleontology of the Bloomsburg Formation of Pennsylvania and adjacent states: G36. Pennsylvania Geologic Survey, Harrisburg, Pennsylvania.

Stratigraphy

Early American geologists theorized that these beds correlated with

Old Red Sandstone The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also exte ...

found in

Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...

., but the Old Red Sandstone is

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

in age, or much younger than the Bloomsburg. The Bloomsburg rests

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 ...

above the Clinton Group and

Shawangunk Formation The Silurian Shawangunk Formation is a mapped bedrock unit in eastern Pennsylvania, New Jersey, and New York. It is named for the Shawangunk Ridge for which it is the dominant rock type. The division of the Shawangunk between the Tuscarora Fo ...

, and has a complex upper contact. In eastern Pennsylvania and New Jersey, the upper part of the Bloomsburg marks the Wallbridge Unconformity. To the west, the

Wills Creek Formation Wills Creek Formation is a mapped Silurian bedrock unit in Pennsylvania, Maryland, Virginia, and West Virginia. Description The Wills Creek is defined as a moderately well bedded greenish-gray shale containing local limestone and sandstone zon ...

lies conformably atop the Bloomsburg. Further west, the McKenzie Member of the

Mifflintown Formation The Mifflintown Formation is a geologic formation in Maryland. It preserves fossils dating back to the Silurian period. See also * List of fossiliferous stratigraphic units in Maryland * Paleontology in Maryland The location of the state of M ...

interfingers with the Bloomsburg and eventually, the Bloomsburg pinches out into the Mifflintown.

Fossils

There are numerous fossils found in the Bloomsburg, mostly in the upper parts of the formation. There are trace fossils of early land plants and

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 ...

fossils in the upper marine transitional zones.

Notable Exposures

* Roundtop Hill, Maryland, along railroad grade *

Port Clinton, Pennsylvania Port Clinton is a borough in Schuylkill County, Pennsylvania, United States. Located at the confluence of the Schuylkill and Little Schuylkill rivers, it was a port on the Schuylkill Canal and named after DeWitt Clinton. Geography According to ...

, where bedding is nearly vertical and cleavage is visible *

U.S. Route 22 U.S. Route 22 (US 22) is a west–east route and is one of the original United States highways of 1926, running from Cincinnati, Ohio, at US 27, US 42, US 127, and US 52 to Newark, New Jersey, at U.S. Route 1/9 in the Newark Airport Intercha ...

, 2.3 km northeast of Canoe CreekFaill, R.T., Glover, A.D., and Way, J.H., 1989, Geology and mineral resources of the Blandburg, Tipton, Altoona, and Bellwood quadrangles, Blair, Cambria, Clearfield and Centre Counties, Pennsylvania: Pennsylvania Geological Survey Topographic and Geologic Atlas, 4th series, 86, 209 p., scale 1:24,000 and 1:48,000

Age

Relative age dating of the Bloomsburg places it in the Upper Silurian period, being deposited between 417 and 410 (±5) million years ago.

Economic uses

The Bloomsburg formation is a source of material for the manufacture of brick and other clay products, as well as a local building stone.