Latitude:                       40° 22' 28"N

Longitude:                    80° 05' 33"W

Quadrangle:           Bridgeville 7 1/2'

Age:                             Pennsylvanian

Formation(s):               Monongahela Group, shales above Pittsburgh coal and Pittsburgh sandstone.

Purpose:                      This site offers a good close-up of the base of a sandstone channel and jointing in shales.

Outcrop is adjacent to, and behind volunteer fire department building. outcrop is at parking lot level. Parking available for motor coach. Recommended for all age groups.

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

From Oakland, take any bus to downtown Pittsburgh. Then 41B (Kane Service) to the bus stop nearest the Volunteer Fire Station on Vanadium Road. Outcrop is behind and adjacent to the Fire Station.

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 0.3 mi. then bear right onto entrance ramp to I-376. Stay on this highway through the Fort Pitt tunnels. Go a total of 8.0 mi. and exit onto I-79 south. Go 4.0 mi then exit at the Heidelberg Exit. Bear right at the exit and go 0.4 mi. At the stop light at Eat n’ Park, make a left onto Vanadium Road. Go 0.6 mi, then make a left into the parking lot next to the Volunteer Fire Company building.  Outcrop is behind and adjacent to the building.

See map and figures.

At this site, the mixed dark and light gray shales immediately above the Pittsburgh coal are well exposed. These shales commonly create significant roof fall problems in underground Pittsburgh coal mines. A single large planar joint over 30 meters long dominates the outcrop. The orientation of the joint is 325 degrees, which is perpendicular to the regional Appalachian trend in the area. This joint probably formed during the Late Paleozoic Alleghanian orogeny (mountain-building event) (Evans, 1994). In the subsurface, joints like this are important conduits for groundwater flow.

A channel sandstone with a very sharp erosional base cuts into the shales. The channel sand exhibits spectacular cross bedding. Locally, this sandstone may cut through the entire section of shales above the coal and rest directly on the coal bed, or even cut it out entirely, creating significant mining problems. Here the sandstone consists of medium to coarse-grained sand. However, at other locations, the base of the channel may contain a very coarse-sand or possibly a quartz pebble conglomerate. Also, the base of the channel may contain fragments of shale and altered wood, or casts where plant debris accumulated.

Geologic History: Environment of Deposition:

The shales above the Pittsburgh coal represent infilling of the coal swamp by clay-rich sediment after Pittsburgh coal time. The shales contain uncommon fragments of plant fossils that were probably washed in. The overlying sandstone, the Pittsburgh sandstone, indicates the emplacement of a distributary stream system into the region. The Pittsburgh sandstone is found throughout southwestern Pennsylvania and varies considerably in thickness (up to 60 feet thick in some locations).

Paleogeographic map showing the region during the during Pittsburgh coal time.

Paleogeographic map showing the region during the during Pittsburgh sandstone time.

Thickness of the Pittsburgh Sandstone from Roen and Kreimeyer (1973).

Click on the thumbnails below for pictures of the outcrops:

A view of the base of the sandstone channel with striking cross bedding.

A close-up of the channel base.

A long (approx. 30 m) planar tectonic joint with an orientation of 325/90.

Fragmented fossil plants may be found in the shales.

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.

Evans, M. A., 1994, Joints and decollement zones in the Middle Devonian shales: Evidence for multiple deformation events in the central Appalachian Plateau: Geological Society of America Bulletin, v. 106, p.447-460.

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.

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.

Roen, J. B., and Kreimeyer, D. F., 1973, Preliminary map showing  the distribution and thickness of sandstone in the lower member of the Pittsburgh Formation, southwestern Pennsylvania and northern West Virginia: U. S. Geological Survey Miscellaneous Field Studies Map MF-529, scale 1:250,000.

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)