The Glenshaw Formation, Illustrated by John A. Harper.
A typical section of the Glenshaw formation in Western Pennsylvania – Illustration by J.A. Harper

The Glenshaw Formation is where I find most fossils featured on this website. N. K. Flint named it in 1965 for its type area at Glenshaw, in the Pine Creek valley of Shaler Township, Allegheny County, just north of Pittsburgh. No single exposure shows the whole formation. The Glenshaw and the younger Casselman Formation comprise the Conemaugh Group, part of an entire third-order T-R unit.

Locally, the top of the formation occurs on hilltops. You’ll find this boundary by encountering the extensive marine stratum known as the Ames Limestone. At lower elevations toward the Kiskiminetas River, the Glenshaw and the Allegheny Formation meet at the Upper Freeport Coal/Lower Mahoning Sandstone boundary.

The Glenshaw Formation contains both marine and plant fossils in Western Pennsylvania. Limestone deposits in the marine zones represent times when ancient seas covered the land and deposited rocks. Limestone forms in clear, warm, shallow marine waters. The resulting layers read like a story.

The Ames Limestone caps the formation. Below it, in descending order, are the Woods Run, Pine Creek, and Brush Creek limestones. The formation begins just above the Upper Freeport Coal, which forms the top of the underlying Allegheny Formation.

Several older research papers refer to the Pine Creek Limestone as the “Upper Brush Creek,” a name still used in Ohio. That usage reflects a real correlation. Conodont studies (Heckel, Barrick, and Rosscoe, 2011) match the Pine Creek Limestone of Pennsylvania with the Upper Brush Creek Limestone of Ohio, and the Brush Creek Limestone of Pennsylvania with only the Lower Brush Creek of Ohio. The Cambridge Limestone of Ohio, long equated with the Pine Creek, instead correlates with the Nadine Limestone of Pennsylvania (Busch and Brezinski, 1984; Heckel, Barrick, and Rosscoe, 2011).

Rock layers of Parks Township Simplified stratigraphic column, youngest at the top: Monongahela Group, Casselman Formation, Glenshaw Formation with the Ames, Woods Run, Pine Creek and Brush Creek limestones, Allegheny Formation, and Pottsville Formation. Not to scale. ▲ younger Monongahela Group Pittsburgh coal Casselman Formation Casselman Fm. ◀ You are here Glenshaw Formation Glenshaw Fm. ◀ You are here Woods Run limestone Pine Creek Limestone Pine Creek Limestone ◀ here Brush Creek Limestone Brush Creek Limestone ◀ here Ames Limestone Ames Limestone ◀ here Upper Freeport coal Allegheny Formation Allegheny Fm. ◀ You are here Brookville coal Pottsville Fm. Conemaugh Group Late Pennsylvanian Middle Pennsylvanian ▼ older · not to scale
Rock layers of Parks Township, simplified and not to scale. Select a layer to read more.

Fossils in the Glenshaw Formation

The Glenshaw in Pennsylvania represents a very dynamic period of sediment deposits. These dynamic deposits produce a wide range of fossils; the rarest are tetrapods. Most vertebrate fossils come from thin, nonmarine limestones or paleosols. A college student at the time, Adam Striegel, discovered the carnivorous tetrapod Fedexia striegeli in the younger Casselman Formation. The paper describing this rare find (Berman et al., 2010) also discusses other vertebrate discoveries within the Conemaugh Group.

With this wide range of available fossils, many types are present. Land vertebrates (extremely rare), fish, plants, and a wide range of marine creatures are available.

Plant Fossils Available

Thus far, I have found many plant fossils between the Woods Run and Brush Creek Limestone. Within, there are various shale and sandstone layers. Plants found locally in the Glenshaw Formation include Calamites, the seed fern Macroneuropteris, and Pecopteris, foliage borne mostly by the tree fern Psaronius. A piece of lepidodendrid scale-tree bark (cf. Lepidophloios) turned up loose on road fill nearby, but its original layer is unknown. Scale trees of this group largely disappeared from the Appalachian Basin at about the time the Glenshaw began to form (DiMichele et al., 2023), so the piece most likely came from older rocks.

Pecopteris, CG-0093
Pecopteris, CG-0093

Marine Fossils Available

Along with plant fossils, marine fossils have been plentiful in my part of the Glenshaw. Fossils occur in limestone, shale, and sandstone, although so far I have found only plant fossils in the sandstone. Examples include Gastropods, Cephalopods, Brachiopods, Crinoids, Clams, Sea Pens, and even a Fish/Shark Tooth.

Certain layers of the Brush and Pine Creek limestones appear to preserve shell material in its original aragonite form.

Solenochilus Fossil Specimen
Above is a Solenochilus, a fossil cephalopod found in the Brush Creek limestone within the Glenshaw Formation.

Geologic Time

This formation represents 5 million years, according to Shulik’s 1979 Ph.D. dissertation. Additionally, Perera and Stigall (2018) show in Figure 2 that the Glenshaw deposits begin at 307 million years ago and end at 303.7 million years ago. These newer estimates place this closer to 3.3 million years. That span corresponds to the Kasimovian Stage of the Late Pennsylvanian, roughly the Missourian Stage in North American terms.

Larger Picture of the Glenshaw

All rocks in the Glenshaw Formation are sedimentary. An ash-fall layer, still part of the sediment, may sit below the Pine Creek limestone (Harper, personal comm.). Several charts in the literature on the formation correlate the various layers with geologic time. This illustration below is a compromise between charts by J.A. Harper and S. N. Perera.

The Glenshaw Formation
An illustrated view of the Glenshaw Formation.
Outcrop of thinly layered dark gray shale in the Glenshaw Formation, with weeds on the ledges and broken shale piled at its base
Shale Cut
Split slab of gray shale with an orange-brown iron-stained patch, surrounded by broken shale fragments

Split Shale
Weathered shale piece showing concentric bands of tan, orange-brown and dark gray coloration among wet shale slabs

Showing Coloration in Shale
Split piece of gray shale with a rounded rust-brown iron-stained surface, lying among broken shale

Shale
Sandstone with underlying shale
Sandstone with Underlying Shale
Sandstone with underlying shale

Sandstone with Underlying Shale
Clay layer
Shale with Clay Layer
Ledge of gray limestone jutting out above thin layers of dark shale on a wooded bank, with ferns and fallen branches
Limestone Outcrop

References and Further Reading

Cross-section of Parks Township

Cross-section of the rock layers under Parks Township Simplified geologic cross-section about two and a half miles long, after J. A. Harper (2019). From the bottom up: Pottsville Formation, Brookville coal, Allegheny Formation, Upper Freeport coal, Glenshaw Formation with the Brush Creek, Pine Creek, Woods Run and Ames limestones, and the Casselman Formation, which survives only as caps on the highest hilltops. Vertical scale exaggerated about five times. Casselman Formation Glenshaw Formation Allegheny Formation Pine Creek Limestone Brush Creek Limestone Ames Limestone Glenshaw Formation Allegheny Formation Pottsville Formation Casselman Formation Ames Limestone Woods Run limestone Pine Creek Limestone Brush Creek Limestone Upper Freeport coal Brookville coal Kiskiminetas River 1,000 ft 1,200 ft 01 mile After J. A. Harper (2019) · vertical scale exaggerated about 5×
Rock layers beneath Parks Township along a line about two and a half miles long, after J. A. Harper (2019). The Brush Creek, Pine Creek and Woods Run limestones are drawn at their approximate positions within the Glenshaw. Select a layer to read more.