Latitude:                           40° 21' 04"N

Longitude:                        80° 02' 59"W

Quadrangle:             Bridgeville 7 1/2'

Age:                                 Pennsylvanian

Formation(s):                   Uppermost Monongahela Group.

Purpose:                           This site provides easy access to freshwater limestones near the top of the Monongahela Group.

Outcrop is at the rear of the Medi-Quick paring lot, slightly above parking lot level. Parking available for motor coach. Seek permission from Medi-Quick for large groups. 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 to Painter’s Run stop. Outcrop is approximately 1000 feet east on Painter’s Run Road. 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 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 1.3 mi, then make a right onto Bower Hill Road. Go 0.3 mi. and at the light, make a left onto Painters Run Road. Go 2.3 mi., then make a left turn at the stop light onto Rt. 19 South. Go 600 feet, make a right at Chi Chi's restaurant and follow road up to intersection with Rt. 19. At the stoplight, make a left turn onto Rt. 19 North. Go 1000 feet and turn right into Medi-Quick parking lot. Park in back next to outcrop.

This is a typical exposure of Upper Pennsylvanian freshwater limestones. The lower section of the outcrop, just above the back wall of the paring lot, includes approximately 2 meters of limestone, overlain by a one meter thick slaly/coaly interval (the Uniontown coal?), which is in turn overlain by 3 meters of flaggy sandstone (the Uniontown sandstone?).  

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, large fresh to brackish water lakes would develop in the inter-distributary parts of the delta. According to Harper (1990), deposition in these large lakes involved carbonate precipitation by algae or other organisms. The conspicuous laminations (layering) that can be seen in many limestone layers is attributed to algal growth that occurred in extensive mats. Also found in these limestones are common breccia-conglomerates that may have formed by periodic drying of the lakebed and the formation of desiccation cracks. Long periods of exposure to weathering processes broke apart the lime beds forming breccias. These breccias were then covered with additional lake sediments as the lakes refilled (Berryhill and others, 1971)

The influx of silt and clay that did occur resulted in alteration of thicker carbonate and thinner non-carbonate muds that lithified into the limestones and shales that can be seen in this outcrop. A modern day analogue for these large lakes might be Lake Ponchartrain in the Mississippia delta region.

Below is a satellite image of the Lake Pontrarchain area.

Paleogeographic map showing the region during the during Benwood limestone time. Highly schematic.

Click on the thumbnails below for pictures of the outcrops:

A close-up of two of the limestone beds

A view of the outcrop

Fragments of fossil plants may be found in the shales and coaly horizon. In the limestones, a typical complement of freshwater ostracodes and rare small pelecepods may be found, along with occasional fish scales and the rare teeth.

Berryhill, H. L., Jr., Schweinfurth, S. P., and Kent, B. H., 1971, Coal-bearing Upper Pennsylvanian and Lower Permian Rocks, Washington area, Pennsylvania: U. S. Geological Survey Professional Paper 621, 47p.

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.

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

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.

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)