Latitude:                             40° 27' 35"N

Longitude:                           80° 10' 21"W

Quadrangle:                       Oakdale 7 1/2'

Age:                                           Pennsylvanian

Formation(s):                     Conemaugh Group, Casselman Formation, Morgantown sandstone

Purpose:                            This site provides an up close look at a large channel sandstone.

Outcrop is along the road to the Federal Express office building. Park along road. Room for a motor coach. Recommended for all age groups.

The above map does not show present road configuration.

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

From Oakland, take any bus to downtown Pittsburgh. Then take 28X or 25D to Robinson Town Center and walk to outcrop (down road past Costco) OR take 25D (Federal Express route) to the Mountour Run Road and walk to outcrop along entrance road to Federal Express. 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. In Downtown Pittsburgh, bear left to I-279 South. Stay on this highway through the Fort Pitt tunnels. From the end of the Fort Pitt Tunnel, go 9.2 miles and exit at the Robinson Town Center exit. At the Stop light at the end of the exit ramp go right, follow the road to a left turn that goes down a hill past Costco. Follow that road to Montour Run Road. At the light, go straight and park alongside the road adjacent to the outcrop.

See map and figures.

This is one of the largest single, easily accessible outcrops in the region. It nicely displays a section through a large sandstone channel, from the sharp erosional base, through the fining upward sequence, and ending in the overlying shale section. The base of the channel sandstone is marked by a polymict conglomerate, which is unusual for this area. The channel cuts deeply into the underlying freshwater limestones and shales. The sandstone exhibits excellent cross bedding and a fining upward sequence.

Geologic History: Environment of Deposition:

Here, the Morgantown sandstone is a distributary channel that has cut into pre-existing freshwater limestones and distributary mouth bar deposits. The setting is lower delta plain, in a sequence that is prograding over the Ames marine unit which underlies the sandstone here by less than 50 feet.

Lower Delta Plain Environment (from Horne and others, 1978).

Typical Sections from the lower delta plain, bottom section with crevass splay (from Horne and others, 1978).

Typical Sequence from lower delta plain (from Horne and others, 1978).

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 (see photo below). 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.

Click on the thumbnails below for pictures of the outcrops:

The eastern portion of the outcrop, corrected for vertical perspective (an true vertical section).

A view of the outcrop from half-way up the hill. Note the difference in color between the sandstone channel (tan) and the underlying shales (dark gray) as seen in the above photo.

Conglomerate at the base of the channel sandstone. US quarter for scale.

The Morgantown sandstone exhibits excellent cross-bedding. Note the ironstone concretions.

Limestone with desiccation(?) cracks

A view of shales above the sandstone.

Fragments of fossil plants may be found in the shales near the base of the outcrop.

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.

Horne, J. C., Ferm, J. C., Caruccio, F. T., and Baganz, B. P., 1978, Depositional models in coal exploration and mine planning in Appalachian region: American Association of Petroleum Geologists Bulletin, v. 62, p. 2379-2411.

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)