Bald Eagle Formation

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Bald Eagle Formation
Stratigraphic range: Late Ordovician
Baldeagle1.JPG
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]

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

Description[edit]

Close-up of outcrop of Bald Eagle Formation, where Rt. 164 crosses Tussey Mountain, Blair County, Pennsylvania. Note prominent crossbedding and lens cap for scale.

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]

Depositional environment[edit]

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.

Fossils[edit]

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.

Age[edit]

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

Economic Uses[edit]

Pyrite in the Bald Eagle Formation, from the Skytop Roadcut where I-99 crosses Bald Eagle Mountain, Centre County, Pennsylvania. Scale bar is 1 cm.
Natural outcrop of the Bald Eagle Formation visible from U.S. Route 522 west of Shade Gap, Huntingdon County, Pennsylvania

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.

References[edit]

  1. ^ 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
  2. ^ 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.
  3. ^ 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.
  4. ^ Berg, T.M., et al., (1983). Stratagraphic Correlation Chart of Pennsylvania: G75, Pennsylvania Geologic Survey, Harrisburg, Pennsylvania.
  5. ^ Doden, Arnold G. and Gold, David P. (2008). "Bedrock Geologic Map of The Mc Alevys Fort Quadrangle, Huntingdon, Centre, and Mifflin Counties, Pennsylvania" (pdf). Pennsylvania Geological Survey. 

See also[edit]

A panorama of the Bald Eagle portion of the Narrows roadcut in Bedford County for the Pennsylvania Turnpike. Stratigraphic up is to the left, and thus the beds are overturned. The beds are highly folded and faulted at right. February 2001.