Sedimentation in Western

Pittsburgh lies in a geographic region called the "Appalachian Plateau", which is a topographically high region west of the Appalachian Mountain chain. The plateau is made up of erosional remains of a large sediment-filled basin which was formed and, finally, uplifted as a result of the plate tectonic interactions which created the Appalachian Moutains.

Pittsburgh sits on over 16,000 feet (OVER 3 MILES) of sedimentary rock. The sedimentary rocks generally seen at the surface in road cuts or outcrops in Pittsburgh were deposited during the Pennsylvanian Period (320 - 290 million years ago), late in the Paleozoic Era. These rocks are among the last to have been deposited in the area. Older sedimentary rocks are buried beneath the surface, only visible in drill holes or in the few places where rivers cut down through the stack of rock units, such as in Conemaugh Gorge near Johnstown.

Outcrops in the Pittsburgh area mostly are part of a rock sequence known as the Conemaugh Group.

This picture shows cross stratification in a sandstone outcropping in Bloomfield. Cross stratification form from moving sand, pushed either by water or air. The curved lower surface of these sandstone beds suggests deposition in a river or the channelized part of a delta . This outcrop is located just across the bridge at the Pittsburgh Brewery as you turn from Liberty Avenue onto Herron Avenue. This outcrop is the Morgantown Sandstone, part of the Casselman Formation of the Conemaugh Group. (site 7 on the map)

This shows an outcrop in Pittsburgh's Strip District, directly east of downtown Pittsburgh off of Liberty Avenue. The outcrop displays cross stratification in sandy deposits. Looking at this and the Bloomfield deposit which is just up the road (across the bridge at the Pittsburgh Brewery) it is easy to see that this area was part of the river-dominated part of a delta. This outcrop is the Morgantown Sandstone, part of the Casselman Formation of the Conemaugh Group.(see site 7 on the map)

More fluvial deposition in Pittsburgh area. This is the RPS road cut in Moon Township just off of Montour Run Road. This roadcut shows a lot of cross stratification, filled channels, and black, organic-rich layers indicating that the rocks are fluvial type deposits.(see site 1 on the map)

Fluvial type cross stratification in Oakland. To find this outcrop drive between the Carnegie Museum and the Frick Art Building. Drive away from Oakland passing over the Panther Hollow Bridge. Make the first right which is located immediately after the bridge. Stay to the right. The outcrop is on the left hand side as you drive towards the City of Pittsburgh's Maintenance and storage area. The outcrop continues along the trail to the lake. The Morgantown Sandstone is at the top, with the Birmingham Shale underneath. (Site 3 on the map)

Geologists use marker beds to help identify which sedimentary layers they are looking at. Marker beds are strata that are very distictive and stand out. A marker bed may have many specific fossils and/or be of a specific type of rock. An important marker bed in the Pittsburgh area is the Ames Limestone. The Ames Limestone has more fossils in it than any other layer in the Pittsburgh area. Fossils of Echinoderms, Brachiopods, and Gastropods are found throughout the Ames. The Ames is the boundary between the Glenshaw and Casselman Formations, both part of the Conemaugh Group. The sedimentary rocks that are exposed in the Pittsburgh area and lay above the Ames layer generally show much cross stratification (more of a fluvial environment). The sedimentary rock layers that are exposed directly below the Ames limestone layer in the Pittsburgh area have been deposited mostly as bay-filling muds and other low energy type depositions related to the seaward part of the delta. (Site 4 on the map)

Frick Park is a great place to find the Ames Limestone layer. Frick Park is located on the East side of Pittsburgh. Follow Forbes Avenue through Squirrel Hill until you see Frick Parks tennis courts on the right. Turn right at the tennis courts and Park in the Parking lot beside the tennis courts. There is a walking trail which has a entrance from the parking lot. Walk down the trail until it comes to a intersection. Make a left at the intersection. The Ames layer is exposed a little way down at the bridge on the next left. A shelter and water fountain are found in this area. Walk away from the bridge and past the water fountain (heading west). The trail goes up a slight grade. Walk up the hill a hundred yards or so and the Ames Limestone can be found on the right and then the larger outcrop is on the left with many fossils exposed. The larger oucrop on the left is a hard ledge in the stream bed, and forms a waterfall when it rains.


Pennsylvanian Period