Rock History-Sep. 1999

There’s History in Mt. Lebanon’s Rocks

As most people drive down Painters Run Road or go through the drive-thru at McDonald’s on Mt. Lebanon Blvd., they rarely notice the rock outcroppings on the side of the road. However, the geologist sees an incredible history in those same rocks.

The rocks here, as with most rocks in Pennsylvania, are the kind geologists call ‘sedimentary.’ That is, they were formed from sediment (clay, silt, sand, gravel, sea shells, plant debris) that accumulated in some ancient environment such as a lake, swamp, beach, coral reef, etc. These environments are assumed to be the same as their modern equivalents. Therefore, by comparing the texture, composition, and fossils in a rock, with sediment being deposited in modern environments, the geologist can determine the depositional environment of the rock, the ancient climate, and the types of plants and animals present when the sediment was deposited.

The sedimentary rocks in the Mt. Lebanon area were deposited between 268 to 296 million years ago during the Pennsylvanian and Permian periods. During that time, the Earth was a very different place because of plate tectonics, the process by which continental and oceanic plates move across the Earth’s surface. The Pittsburgh area and Eastern North America were located about 6 degrees south of the equator. The climate was similar to present-day New Guinea, hot, humid, and tropical. Vast swamps and forests covered large delta regions in what is now western Pennsylvania, and the open sea lies only as far as Ohio. In the central Pennsylvania area, the ancient Appalachian mountains rose to the height of the present-day Andes. Rain, ice and wind constantly eroded these mountains, sending sediments down vast river systems to form the deltas.

How do we know? The evidence is in the rocks. For example, behind McDonald’s the layers of rock include tan to light gray hard limestone and dark gray shales of the Uniontown limestone. The limestones were deposited in large, warm, fresh-water lakes located in the swamps, much like present-day Lake Pontchartrain in the Mississippi delta. Fossils in the limestones are rare, but occasionally one may find a few fish scales or small shells. The shales were deposited when rivers in the swamps overflowed their banks and spread large blankets of silt and clay across the area.

Heading west along Painters Run Road, just before you get to Jim Jenkins garden center, the rock outcrop on the left side of the road includes the massive Pittsburgh Coal bed. Here, the coal is nearly six feet thick. If you break off a piece of the coal and look at it closely, you will see faint traces of plant material. The coal accumulated as plant debris in a swamp environment. The Pittsburgh Coal swamp was very large. At a minimum, it extended from north of Pittsburgh to central West Virginia, and from eastern Ohio to Ligonier. In order to form six feet of coal, nearly 300 feet of plant material accumulated, and was then compacted through time. The shales that overlies the coal record the flooding of the swamp. The Pittsburgh Coal is the lowest rock layer, and therefore the oldest layer, exposed in Mt. Lebanon.

Every rock has a story that adds to our knowledge of Earth history.

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