Latitude:                         40° 20' 42"N

Longitude:                       80° 09' 14"W

Quadrangle:                    Canonsburg 7 1/2'

Age:                                Pennsylvanian

Formation(s):                 Monongahela Group, Pittsburgh Formation, Limestones A-D and Uniontown(?) sandstone at top.  

Purpose:                        This site provides access to a well exposed section of the upper Pittsburgh Formation.

Park along side of road adjacent to outcrop. Parking available for motor coach. Recommended for all age groups. Due to inclined roadway, all parts of the section are accessible at roadside.

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

No PAT Transit Service.

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 to the Bridgeville Exit. Bear right at the exit and at the stop light at the end of the exit ramp, go left of Rt. 50. Go 2.5 miles and take a left. park along guardrail on right side of road. Outcrop is along road.

See map and figures.

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, which is in turn overlain by 3 meters of flaggy 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 Late Pittsburgh time. Highly schematic.

Click on the thumbnails below for pictures of the outcrops:

A view of the outcrop showing interbedded shales and limestones of the Pittsburgh Formation. Note sandstone (Uniontown (?)) at top.

A close-up of a lingula brachiopod.

Weathered surface of a limestone breccia.

A fresh broken surface of a limestone breccia.

Uncommon plant fossils may be found in the shales.

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)