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. It is a type section named by N. K. Flint in 1965. The Glenshaw and the younger Casselman Formation together comprise the Conemaugh Group. The Conemaugh Group is part of an entire third-order T-R unit.

The top part of the formation locally is located at hilltops. This boundary can be found by locating an extensive marine stratum known as the Ames Limestone. If I look at lower elevations towards 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. There are a number of marine zones that may contain limestone deposits. These 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.

Fossils in the Glenshaw Formation

Since the Glenshaw in Pennsylvania represents a very dynamic period of sediment deposits, there is a very wide range of fossils to be found. The rarest seems to be the tetrapods, with limited examples having been found. As simple as I can put it, vertebrates are found either in thin, nonmarine limestones, or paleosols. These are few and far in-between. While Fedexia striegeli was found within the Casselman Formation, the paper describing this rare find describes vertebrate finds within the Conemaugh Group in more detail.

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

Plant Fossils Available

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.

Pecopteris, CG-0093
Pecopteris, CG-0093

Marine Fossils Available

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.

Particular layers of the Brush and Pine Creek limestones appear to preserve shell material with original aragonite material remaining.

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

Geologic Time

This formation represents 5 million years of time, according to a Ph.D. 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 Formation is composed of 100% sedimentary rock. There may be an ash fall layer below the Pine Creek limestone, however, this is still 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