Latitude:                             40° 24' 17"N

Longitude:                         80° 01' 11"W

Quadrangle:                       Pittsburgh West 7 1/2'

Age:                                            Pennsylvanian

Formation(s):                     Monongahela Group - Conemaugh Group, Pittsburgh Coal

Purpose:                            This set of three adjacent outcrops provides a complete section through the Pittsburgh Coal.  Also, good example of mine subsidence.

Park along West Liberty Avenue near site. Ask permission at auto repair shop to view outcrop. Recommended for all age groups.

(Make sure you get an up-to-date PAT Transit schedule:

From Oakland, take a 61A or 61B bus to the corner of Forbes and Braddock avenues. Walk south on Braddock approx. 1000 feet until you come to a parking lot adjacent to the Frick tennis courts. Enter trail here. Return.

From the Cathedral of Learning, Drive 0.7 mi. west on Fifth Avenue. Make a Left on Craft Av. Go 0.1 mi., then make a right onto Blvd. of Allies, go 1.6 mi. then follow signs to Liberty Bridge and into the Liberty Tunnel. Stay in left lane to head south on Truck Rt. 19. From the south side of the Liberty tunnel, go 0.8 miles south on West Liberty Avenue. Park along road across street from Toyota car dealer lot, and walk to outcrop.

See map and figures.

This is an excellent exposure of the Pittsburgh Coal bed, the thickest and most widespread coal in the central Appalachians. In southwestern Pennsylvania the Pittsburgh coal averages 6 to 12 feet thick. A characteristic parting is found here separating the coal into two distinct layers. This shale parting is regionally continuous.

Break a piece of the coal off or find a piece in the talus slope. Take a close look and note that there are different kinds of coal based on the reflectance (how shiny) it is. The shiniest coal is called vitrinite, the dullest (which looks like ash) is called fusinite. Also, note the presence of fossil plant fragments through the coal. The coal itself is highly fractured, containing many joints called coal cleat. There are two kinds of cleat, a dominant face cleat and a butt cleat that is nearly perpendicular to it. The face cleat determines how miners set-up a coal mine because it is the easiest direction to mine along.

The Pittsburgh Coal bed has been almost completely mined out in Allegheny County, PA. This outcrop section contains considerable evidence of this mining. Undulating shales above the coal indicate subsidence of the ‘rooms’ while relatively horizontal sections are above the ‘pillars.’

Geologic History: Environment of Deposition:

During the middle Pennsylvanian, western Pennsylvania was located approximately 5 to 10 degrees south of the equator and had a tropical to subtropical environment. Some geologists suggest that the area was in a similar setting to that of modern-day New Guinea. Western Pennsylvania was the site of a deltaic system that bordered a large shallow sea coving much of the central North America. Sediments were fed into the delta region by large river systems originating in the growing Alleghanian mountains to the east. The mountains were growing because of the continuing convergent and collision of North America and the African portion of the Gondwana supercontinent.

Locally, the shales below the coal were formed from clay and silt deposited in a swamp environment. The coal was formed by the accumulation of plant debris. A general rule of thumb states that for bituminous coal, 30 cm of coal requires 15 meters of plant debris. So, for this site, nearly 125 meters of plant debris accumulated to form the coal. The split in the coal is the result of the shifts of position of a distributary channel in the ancient delta. This caused more clastic sediment to be pumped onto the delta plain, thereby destroying the condition for coal formation. After some time, coal forming condition became re-established and peat accumulated again. At some point, the plant growth ceased and the subsiding swamp was covers with more clays and silt.

Paleogeographic map showing the region during Pittsburgh Coal time.

Field Museum of Natural History in Chicago

 National Museum of Natural History,  New York

for other views of the Coal swamp era go to: http://www.uni-muenster.de/GeoPalaeontologie/Palaeo/Palbot/seite12.html

Click on the thumbnails below for pictures of the outcrops:

A view of the Pittsburgh coal outcrop. Note the two separate benches of the coal.

Close-up of the coal showing the bright and dull layers.

Close-up of the coal bed showing difference between bright (vitrinite) and dull (fusinite, intertinite) coal.

Compaction-related slip surfaces in the Pittsburgh coal bed. US Quarter for scale.

Bloom of ammonium nitrate? on surface of coal.

Well-formed synclinal fold in shales above coal. Formed due to subsidence above old mine shaft.

Section below coal as exposed in open cut outcrop to the north.

Cordaites leaves as fossil imprints in shale. Found in open cut outcrop area to north of main outcrop. Most likely from shales above the Pittsburgh coal. US quarter for scale.

Section of shale above Pittsburgh coal. In outcrop adjacent to Toyota dealership. Upper bench of coal exposed here.

The following is a map showing the regional thickness pattern of the Pittsburgh coal

U.S. Geological Survey Open-File Report 97-748

Many fossil plant fragments may be found in the shales above and below the Pittsburgh Coal. Occasionally, whole plant leaves may be found. Within the coal itself, seventy to eighty different species of plants have been identified. Again, mostly fossil fragments are found.

Cross, A. T., 1971, The Geology of the Pittsburgh Coal, West Virginia Geological Survey Report of Investigations, No. 10., 99 p.

Edmunds, W. E., Skema, V. W., Flint, N. K., 1999, Pennsylvanian, in Shultz, C. H., ed, The Geology of Pennsylvania, Pennsylvania Geological Survey Special Publication 1, p. 149-169.

Gillespie, W. H., Clendening, J. A., and Pfefferkorn, H. W., 1978, Plant Fossils of West Virginia, West Virginia Geological Survey Educational Series ED-3A, 172p.

Harper, J. A., 1990, Fossil Collecting in the Pittsburgh Area, Pittsburgh Geological Society Guidebook, 50 p.

Johnson, M. E., 1928, Geology and Mineral Resources of the Pittsburgh Quadrangle, Pennsylvania: Pennsylvania Geological Survey Bulletin A 27, 236 p.

Leighton, H. 1945, The Geology of Pittsburgh and its Environs: A Popular Account of the General Geological Features of the Region: Carnegie Institute Press, 2nd edition, Pittsburgh, PA , 80p.

Leighton, H. Guidebook to the Geology about Pittsburgh: Pennsylvania Geological Survey Bulletin G 17, 35 p.

Shaw, E. W., and Munn, M. J., 1911, Geologic Atlas of the United States: Burgettstown-Carnegie Folio, United States Geological Survey Folio 177 Field Edition, 123p.

Wagner, W. R., and others, 1970, Geology of the Pittsburgh Area: Pennsylvania Geological Survey General Geology Report G 59, 145p.

Click here for  an image of the County Geologic Map (1880)