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 the primary formation in which I find most fossils featured on this website. It is a type section named by N. K. Flint in 1965. The Glenshaw and the younger Casselman Formation together comprise what is known as the Conemaugh Group. The Conemaugh group is part of an entire third-order T-R unit.

The top part of the formation is located near the local hilltops, where the Ames Limestone starts. Going downhill towards the Kiskiminetas River, the Glenshaw and the Allegheny Formation meet, at the Upper Freeport Coal/Lower Mahoning Sandstone boundary.

The Glenshaw contains both marine and plant fossils in Western Pennsylvania. There are a number of marine zones that correlate with limestone layers that 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 top of the formation. In-between you will find Woods Run Cambridge, Pine Creek, and Brush Creek Limestone. The bottom of the formation is the Upper Freeport Coal layer. Unfortunately, a number of older research papers has the Pine Creek Limestone referenced as the Upper Brush Creek. This designation is also popular in Ohio. It can be confusing to correlate limestone layers between Pennsylvania and our neighbors.

Pecopteris, CG-0093
Pecopteris, CG-0093

Plant Fossils in the Formation

Thus far, I have found a great number of plant fossils between the Woods Run and Brush Creek Limestone. Within, there are various shale and sandstone layers. Examples of plants found locally in the Glenshaw Formation are Calamites, Lepidodendron, Macroneuropteris and the seed fern Pecopteris.

Marine Fossils in the Formation

Solenochilus Fossil Specimen
Solenochilus, a specimen found in the Brush Creek Limestone within the Glenshaw Formation

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

Geologic Time

This formation represents 5 million years of time, according to a PhD dissertation by Shulik 1979. According to Perera and Stigall in 2018, figure 2 of their research shows the Glenshaw deposits starting at 307 million years ago and ending at 303.7 million years ago. These newer estimates are placing this closer to 3.3 million years of time.

Larger Picture of the Glenshaw

The Glenshaw is composed of 100% sedimentary rock. There may be an ash fall layer under the Pine Creek, however this is still a sediment that was laid down and later lithified. In researching the formation, there were several charts I found that 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
The Glenshaw Formation

Gallery of Rock Photos in the Glenshaw Formation

References and Further Reading