Latitude:                             40° 26' 56"N

Longitude:                          80° 10' 23"W

Quadrangle:                       Oakdale 7 1/2'

Age:                                            Pennsylvanian

Formation(s):                     Monongahela Group-Conemaugh Group, Pittsburgh coal. Pittsburgh sandstone at top of outcrop.

Purpose:                            This site provides a good section from the upper Conemaugh Group through the lower Monongahela Group, including the Pittsburgh Coal bed.

Outcrop is at road level. Parking available for motor coach. Park in Wal Mart parking lot or in parking lot near McDonald's restaurant.  Recommended for all age groups.

The above map does not show the present road configuration.

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

From Oakland, take any bus to downtown Pittsburgh. Then take 25D to Robinson Town Center and walk to outcrop. 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 left. Cross over the main highway and turn left at stoplight into Lowe's/Wal Mart parking area. Outcrop is on hillside at top of entrance road. Park at any convenient space.

See map and figures.

This is an excellent exposure of the Pittsburgh Coal bed, the thickest and most widespread coal in the central Appalachians. In southwestern Pennsylvania the Pittsburgh coal averages 6 to 12 feet thick. A characteristic parting is found here separating the coal into two distinct layers. This shale parting is regionally continuous. Just to the east of this site is a small, yet interesting , split in the upper portion of the seam.

Break a piece of the coal off or find a piece in the talus slope. Take a close look and note that there are different kinds of coal based on the reflectance (how shiny) it is. The shiniest coal is called vitrinite, the dullest (which looks like ash) is called fusinite. Also, note the presence of fossil plant fragments through the coal. The coal itself is highly fractured, containing many joints called coal cleat. There are two kinds of cleat, a dominant face cleat and a butt cleat that is nearly perpendicular to it. The face cleat determines how miners set-up a coal mine because it is the easiest direction to mine along.

The Pittsburgh Coal bed has been almost completely mined out in Allegheny County, PA. This outcrop section contains considerable evidence of this mining. Approximately 50 meters east of this outcrop is a concrete plug, sealing a mine opening. Approximately 300 meter to the west, behind the Sunoco gas station is a sealed porthole to the mine. The mine here was called the Essen mine and was active in the early 1900’s. Additional evidence for mining is seen approximately 500 meters east of the outcrop where undulating shales above the coal indicate subsidence of the ‘rooms’ while relatively horizontal sections are above the ‘pillars.’

The limestones below the coal are of the Upper Conemaugh Group.

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, the shales below the coal were formed from clay and silt deposited in a swamp environment. The coal was formed by the accumulation of plant debris. A general rule of thumb states that for bituminous coal, 30 cm of coal requires 15 meters of plant debris. So, for this site, nearly 125 meters of plant debris accumulated to form the coal. The split in the coal is the result of the shifts of position of a distributary channel in the ancient delta. This caused more clastic sediment to be pumped onto the delta plain, thereby destroying the condition for coal formation. After some time, coal forming condition became re-established and peat accumulated again. At some point, the plant growth ceased and the subsiding swamp was covers with more clays and silt.

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 Pittsburgh Coal time.

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

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

Field Museum of Natural History in Chicago

  National Museum of Natural History,  New York

for other views of the Coal swamp era go to: http://www.uni-muenster.de/GeoPalaeontologie/Palaeo/Palbot/seite12.html

Click on the thumbnails below for pictures of the outcrops:

A view of the highest part of the section, the Pittsburgh Sandstone interval. From dirt road behind McDonald's

A view of the middle part of the section, the Pittsburgh Coal is in the talus slope on the bench. From dirt road behind McDonald's

A view of the lowest part of the section, the Upper Conemaugh interval. From dirt road behind McDonald's

A view of the entire section from the Wal Mart parking lot. Note Pittsburgh Coal in middle of section.

The following is a map showing the regional thickness pattern of the Pittsburgh coal

Many fossil plant fragments may be found in the shales above and below the Pittsburgh Coal. Occasionally, whole plant leaves may be found. Within the coal itself, seventy to eighty different species of plants have been identified. Again, mostly fossil fragments are found. The limestones contain the typical assemblage of small to micro ostracodes and pelecepods, along with fish teeth and scales.

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.

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.

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.

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)