Bald Eagle Formation

Bald Eagle Formation
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
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

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

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

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

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.

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]

Economic Uses

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

^ 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. Archived from the original (PDF) on June 9, 2011. {{cite journal}}: Cite journal requires |journal= (help)