The Brush Creek limestone is one of four largely laterally extensive fossiliferous limestone strata within the Glenshaw Formation of the Middle to Upper Pennsylvanian age Conemaugh Group. It is named for outcrops occurring along Brush Creek in Cranberry Township, which is in neighboring Butler County. The limestone is very dark in appearance, being a dark gray rock.

Locally, 100 percent of all limestone fossils we collect likely comes from the Brush Creek. There are neighboring limestones in the stratigraphic layers, such as the Pine Creek and Woods Run limestones. These can be mistaken for Brush Creek (or vice versa) and have been many times in the recent past in Western Pennsylvania. I am still not 100 percent sure I am looking at Brush Creek, but I don’t have any evidence against it yet. According to Hughes (1933), the Upper Freeport coal sits 120 feet below the Brush Creek limestone. Stratigraphic sections from cores taken in Parks Township, however, indicate that the Brush Creek coal, which is stratigraphically lower than the limestone, is consistently about 150 feet above the Upper Freeport.

What is Limestone?

Of the major sedimentary rock types, shale, sandstone, and limestone are the three most prolific types. Shales form when very small bits of sediment, 1/256 mm in size or smaller (mud and silt), become deposited by wind, water, or ice. Sandstones are created in much the same way, except that they are made of much larger pieces of sediment, 1/16 – 2 mm in size.

Marine limestones like the Brush Creek or the Pine Creek are formed in clear, shallow, warm seas. These rocks are made of a mixture of at least 50% calcium carbonate (CaCO3) which is precipitated from the water or accumulated from the shells of dead plants and animals. This mineral is in the form of either calcite or aragonite, which forms the shells. Aragonite has a different crystal structure than calcite and is less stable. Over time, calcite replaces the aragonite structures and features of the fossilized fauna within. It creates an interesting crystal pattern in some fossil specimens I find, often with discernable cleavage planes.

Where did Brush Creek Limestone Come From?

Most local rocks were deposited at or around 304 million years of age. This was a time where sea levels would often rise and fall, at times flooding the area. These are known as glacial-eustatic fluctuations in sea level. Global warming and cooling that occurred during this time caused ice to melt or form. This increased or decreased the eustatic sea level, which is a measurement of the height of the sea from the center of the earth (Patzkowsky & Holland 2012).

Western Pennsylvania is known to have been South of the equator at that time. Meaning that the water was warm, possibly shallow, and likely clear. This happened more than once it seems, as there are several laterally extensive marine limestone layers in the area. The Pine Creek and the Ames are two more examples of times when water extensively flooded the area.

However, these were not quick flooding events. Over the course of tens to hundreds of thousands of years, the water would come in, stay for a while, and eventually leave. The world in the Carboniferous was much different, with a year lasting around 385 days and a day itself was only 22.4 hours (Wells 1966). Fossil corals were used to determine this. These corals have a microscopic growth layer that accumulates each day throughout the year. Scientists counted these layers from fossil specimens and were able to provide estimates for the length of the year (Wells 1966).

How Do We Know The Ages?

Sedimentary rock is really difficult to date. Since it is literally made up of numerous pieces of other rocks all rearranged, getting an accurate date is tough. Furthermore, you can only get accurate dates from certain kinds of rock. There are certain fundamental rules in geologic stratigraphy that are correct most of the time. One of them, the Principal of Superposition, states that layers of rocks are arranged by age. The higher layers are positioned the younger their age. There are some very unusual examples on Earth, however, where entire rock groups are flipped 90 or even 180 degrees.

The radioactive isotope potassium-40 (40K) is common in igneous rocks. It decays at a specific rate to form argon-40 (40Ar) and this can be measured when dating igneous rocks (K-Ar dating). Since 50 percent of the potassium-40 will have turned into argon-40 in approximately 1.2 billion years, scientists can calculate the age of these rocks. This is helpful if a volcano has deposited rocks in a layer near the sedimentary rock you want to date.

However, this doesn’t help us here in this area of Pennsylvania. While there are igneous rocks in the state, there do not appear to have been any volcanoes depositing rocks in the local layer adjacent to the Brush Creek limestone. There is a possible layer of volcanic ash in the Glenshaw Formation, but it has not yet been properly researched.

Carboniferous timeline showing various chronostratigraphic stages.
The Carboniferous timescale with various chronostratigraphic stages. The Brush Creek limestone is from the Kasimovian stage.

Ways to Correlate Sedimentary Strata

There is another way to correlate rock strata over long distances using fossil evidence. The Brush Creek and Pine Creek limestones are quite similar to the casual observer. An attempt to identify and correlate these layers by fossil species has been tried several times using macrofossils. And although it can be a very successful process in some instances, the Brush Creek and Pine Creek macrofossils generally are so similar as to be useless for correlation. For example, there have been no examples of the genus Tainoceras recovered below the Ames and Woods Run Limestone. However, to correctly correlate a particular stratum using macrofossils, it’s best to use a series of particular species.

There is where conodonts come in. These microscopic fossil oral elements that resemble teeth from extinct eel-like vertebrates are extremely useful for identifying and correlating sedimentary layers. Because they also evolved into different species quickly, the conodonts components in the rock changed rapidly. Different conodonts can be found in each of the marine limestones in western Pennsylvania, allowing scientists to correlate the local rocks with those throughout North America.

LimestoneConodont Species
Amesldiognathodus simulator
Woods RunStreptognathodus gracilis
NadineStreptognathodus gracilis
Pine Creekldiognathodus confragus
Brush Creekldiognathodus cancellosus
Conodont index species across local Western PA Limestone (Heckel et al., 2011)

Examples of Brush Creek limestone

Polished piece of Brush Creek limestone
Crinoids and other fossil marine life in a specimen of Brush Creek limestone. It has been polished to 1,500 grit to enhance the color.
Coral in Polished Brush Creek limestone
Polished piece of Brush Creek limestone. A horn coral species is seen in the top right of the specimen.
Brush Creek limestone exposed in a hillside.
Brush Creek limestone in situ. A tree has taken root above and is pushing it down.

Brush Creek Limestone Compared to the Ames Limestone

The Ames Limestone is a younger limestone than the Brush Creek with a separation of approximately 2.3 million years of time between the two ancient seas. The Ames has a much lighter matrix, likely due to a less anoxic environment when the sea invaded the land. As the marine creatures died, they settled on the bottom of the sea. With more oxygen in the water, their organic parts decomposed more easily via oxidation.

Interesting to note, the cephalopod Tainoceras can be found in the Ames Limestone but is absent from the Brush Creek limestone.

Brush Creek limestone next to Ames Limestone
Brush Creek limestone (left) next to Ames Limestone (right). Notice the lighter color of the Ames. Both contain many fossils, including some that are common to both limestones.


I would like to extend a great thanks to John A. Harper for giving me a peer-review of this article and for educating me about western Pennsylvania geology.