Latitude:                         40° 21' 52"N

Longitude:                       80° 01' 14"W

Quadrangle:                         Bridgeville 7 1/2'

Age:                                Pennsylvanian

Formation(s):                  Monongahela Group, Pittsburgh Formation, Benwood limestone

Purpose:                         This site offers a close-up view of the Benwood limestone and some unusual mound-like features.

Outcrop is at road level. Parking available for motor coach. Park adjacent to abandoned gas station. Keep clear of traffic in new gas station.

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

From Oakland, take any bus to downtown Pittsburgh. Then 41A or 41F to Rt. 88 in Castle Shannon.  Get off at stop nearest Sunoco Gas Station (Across Rt. 88 from Arby's Restaurant). Outcrop is in parking lot adjacent to gas station. 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 2.5 miles south on West Liberty Avenue. Make a left onto Scott Road (Scott Road merges with Castle Shannon Boulevard). Go 2.0 miles. then, make a left onto Rt. 88 North in Castle Shannon. Go 0.12 miles and turn into gas station on right. Park here. Outcrop is in parking lot adjacent to the gas station.

See map and figures.

This is an easily accessible outcrop of portion of the Benwood limestone section of the Monongahela Group. These are fine-grained clay-rich limestones that occur in beds up to two feet thick with intervening shale layers. Here, the beds contain several mounds (see photos below) that do not appear to be biogenic. However, closer examination may reveal otherwise.

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.

Click on the thumbnails below for pictures of the outcrops:

A view of part of the outcrop and one of the smaller limestone mounds.

A close-up of one of the limestone mounds.

Shales contain common plant fragment fossil. Limestones contain common ostracodes and rare fish teeth. Look in silty intervals for fish scales, ostracodes and 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)