Bald Eagle Formation

Bald Eagle Formation
Stratigraphic range: Late Ordovician
Bald Eagle Formation; Stratigraphic range: Late Ordovician
	Outcrop of Bald Eagle Formation, where Rt. 164 crosses Tussey Mountain, Blair County, Pennsylvania
Type	sedimentary
Underlies	Juniata Formation
Overlies	Reedsville Formation
Thickness	275 +/-25 m [1]
Lithology
Primary	sandstone
Other	conglomerate, shale
Location
Region	Appalachian Mountains
Extent	Pennsylvania, Maryland, West Virginia
Type section
Named for	Bald Eagle Mountain, Pennsylvania
Named by	A. W. Grabau [2]

Last updated March 12, 2019

The Ordovician Bald Eagle Formation is a mapped bedrock unit in central Pennsylvania, USA. It is a ridge-forming unit in the Appalachian Mountains.

The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya.

Pennsylvania, officially the Commonwealth of Pennsylvania, is a state located in the northeastern and Mid-Atlantic regions of the United States. The Appalachian Mountains run through its middle. The Commonwealth is bordered by Delaware to the southeast, Maryland to the south, West Virginia to the southwest, Ohio to the west, Lake Erie and the Canadian province of Ontario to the northwest, New York to the north, and New Jersey to the east.

A cliff-former is a geological unit of bedrock that is more resistant to erosion than overlying or underlying strata and consequently produces outcrops with high slope angles. It is more or less equivalent to ridge-former, and may be contrasted with slope-former. In humid environments, sandstones are typically cliff-formers. In arid environments, limestones are often cliff-formers also. Recent lavas may be cliff-formers as well.

Description

The Bald Eagle is defined as a gray to olive-gray and grayish-red, fine to coarse-grained crossbedded sandstone or greywacke. A conglomeratic member, called the Lost Run Member, exists in some locations.^[3]

Sandstone is a clastic sedimentary rock composed mainly of sand-sized mineral particles or rock fragments.

Greywacke A hard, dark sandstone with poorly sorted angular grains in a compact, clay-fine matrix

Greywacke or graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found in Paleozoic strata. The larger grains can be sand- to gravel-sized, and matrix materials generally constitute more than 15% of the rock by volume. The term "greywacke" can be confusing, since it can refer to either the immature aspect of the rock or its fine-grained (clay) component.

Conglomerate is a coarse-grained clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts, e.g., granules, pebbles, cobbles, and boulders, larger than 2 mm (0.079 in) in diameter. Conglomerates form by the consolidation and lithification of gravel. Conglomerates typically contain finer grained sediment, e.g., either sand, silt, clay or combination of them, called matrix by geologists, filling their interstices and are often cemented by calcium carbonate, iron oxide, silica, or hardened clay.

Depositional environment

The depositional environment of the Bald Eagle has always been intrepreted as mostly terrestrial or shallow marine deposits resulting in a molasse sequence produced by the Taconic orogeny.

Molasse Sandstones, shales and conglomerates that form as terrestrial or shallow marine deposits in front of rising mountain chains.

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 flysch—like, for example, those that left from the rising Alps, or erosion in the Himalaya. These deposits are typically the non-marine alluvial and fluvial sediments of lowlands, as compared to deep-water flysch sediments. Sedimentation stops when the orogeny stops, or when the mountains have eroded flat.

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 United States. As the mountain chain eroded in the Silurian and Devonian periods, sediments from the mountain chain spread throughout the present-day Appalachians and midcontinental North America.

Fossils

Very few fossils exist in the Bald Eagle Formation, and most of them are trace fossils. However, at the base of the formation is the Orthorynchula biostratigraphic marker bed, which contains abundant Orthorynchula brachiopods.

A trace fossil, also ichnofossil, is a geological record of biological activity. Ichnology is the study of such traces, and is the work of ichnologists. Trace fossils may consist of impressions made on or in the substrate by an organism: for example, burrows, borings (bioerosion), urolites, footprints and feeding marks, and root cavities. The term in its broadest sense also includes the remains of other organic material produced by an organism — for example coprolites or chemical markers — or sedimentological structures produced by biological means - for example, stromatolites. Trace fossils contrast with body fossils, which are the fossilized remains of parts of organisms' bodies, usually altered by later chemical activity or mineralization.

Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. Usually the aim is correlation, demonstrating that a particular horizon in one geological section represents the same period of time as another horizon at some other section. The fossils are useful because sediments of the same age can look completely different because of local variations in the sedimentary environment. For example, one section might have been made up of clays and marls while another has more chalky limestones, but if the fossil species recorded are similar, the two sediments are likely to have been laid down at the same time.

Age

Relative age dating of the Bald Eagle places it in the Upper Ordovician period, being deposited between 488.3 and 443.7 (±10) million years ago. It rests conformably atop the Reedsville Formation and conformably below the Juniata Formation.^[4]

The Ordovician Reedsville Formation is a mapped surficial bedrock unit in Pennsylvania, Maryland, Virginia, West Virginia, and Tennessee, that extends into the subsurface of Ohio. This rock is a slope-former adjacent to the prominent ridge-forming Bald Eagle sandstone unit in the Appalachian Mountains. It is often abbreviated Or on geologic maps.

The Ordovician Juniata Formation is a mapped bedrock unit in Pennsylvania and Maryland. It is a relative slope-former occurring between the two prominent ridge-forming sandstone units: the Tuscarora Formation and the Bald Eagle Formation in the Appalachian Mountains.

Economic Uses

The Bald Eagle is a good source of road material, riprap and building stone.^[5] However, iron pyrite inclusions may lead to acidic rainwater runoff.

Riprap rock used for armouring structures

Riprap, also known as rip rap, rip-rap, shot rock, rock armor or rubble, is rock or other material used to armor shorelines, streambeds, bridge abutments, pilings and other shoreline structures against scour and water or ice erosion. Common rock types used include granite and limestone. Concrete rubble from building and paving demolition is sometimes used.

Related Research Articles

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 Formation and Clinton Group has not been conclusively determined. The shift of nomenclature currently has the divide between Hawk Mountain and Lehigh Gap.

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 red sedimentary rocks in the Appalachian Basin.

The Ordovician Martinsburg Formation (Om) is a mapped bedrock unit in Pennsylvania, New Jersey, Maryland, Virginia, and West Virginia. It is named for the town of Martinsburg, West Virginia for which it was first described. It is the dominant rock formation of the Great Appalachian Valley in New Jersey (where it is called Kittatinny Valley and Pennsylvania.

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 Passaic, New Jersey, which is near where its type section was described by paleontologist Paul E. Olsen.

The Llewellyn Formation is a mapped bedrock unit in eastern Pennsylvania. It was previously known as the "coal measures" and the post-Pottsville rocks. The formation is named for the community of Llewellyn in Schuylkill County.

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.

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The Devonian Hamilton Group is a mapped bedrock unit in the United States. The unit is present in New York, Pennsylvania, Maryland, Ohio and West Virginia. In Virginia, it is known as the laterally equivalent Millboro Shale. The group is named for the village of Hamilton, New York. Details of stratigraphic nomenclature for this unit as used by the U.S. Geological Survey are available on-line from the National Geologic Map Database. These rocks are the oldest strata of the Devonian gas shale sequence.

The Devonian Mahantango Formation is a mapped bedrock unit in Pennsylvania, West Virginia, and Maryland. It is named for the North branch of the Mahantango Creek in Perry and Juniata counties in Pennsylvania. It is a member of the Hamilton Group, along with the underlying the Marcellus Formation Shale. South of Tuscarora Mountain in south central Pennsylvania, the lower members of this unit were also mapped as the Montebello Formation. Details of the type section and of stratigraphic nomenclature for this unit as used by the U.S. Geological Survey are available on-line at the National Geologic Map Database.

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The Mississippian Mauch Chunk Formation (Mmc) is a mapped bedrock unit in Pennsylvania, Maryland, and West Virginia. It is named for the township of Mauch Chunk, now known as borough of Jim Thorpe, Pennsylvania and for nearby Mauch Chunk Ridge where the formation outcrops.

The Cambrian Chickies Formation is a mapped bedrock unit in Pennsylvania, New Jersey, and Maryland. It is named for Chickies Rock, north of Columbia, Pennsylvania along the Susquehanna River.

The Lock Haven Formation is a Devonian mapped bedrock unit in Pennsylvania, in the Appalachian Mountains of the United States.

The Devonian Old Port Formation is a mapped bedrock unit in Pennsylvania, USA. Details of the type section and of stratigraphic nomenclature for this unit as used by the U.S. Geological Survey are available on-line at the National Geologic Map Database. Current nomenclature usage by U.S. Geological Survey restricts the name Old Port Formation to Pennsylvania, but correlative units are present in adjacent states.

The Triassic Stockton Formation is a mapped bedrock unit in Pennsylvania, New Jersey, and New York. It is named after Stockton, New Jersey, where it was first described. It is laterally equivalent to the New Oxford Formation in the Gettysburg Basin of Pennsylvania and Maryland.

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Egg Hill is an elongate hill trending northeast-southwest in southern Centre County, Pennsylvania. It is mostly forested and uninhabited. Sinking Creek flows northeastward along the northern foot of Egg Hill. The town of Spring Mills is located at the northeastern end.

References

↑ Faill, 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
↑ Grabau, A.W., 1909, Physical and faunal evolution of North America during Ordovicic, Siluric, and Early Devonic time: Journal of Geology, v. 17, p. 209-252.
↑ 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.
↑ Berg, T.M., et al., (1983). Stratagraphic Correlation Chart of Pennsylvania: G75, Pennsylvania Geologic Survey, Harrisburg, Pennsylvania.
↑ Doden, Arnold G. & Gold, David P. (2008). "Bedrock Geologic Map of The Mc Alevys Fort Quadrangle, Huntingdon, Centre, and Mifflin Counties, Pennsylvania" (pdf). Pennsylvania Geological Survey.