The Brush Creek Limestone is one of two largely laterally extensive fossiliferous limestone strata within the Conemaugh Group. It is named for outcrops occurring along Brush Creek in Cranberry Township, which is in neighboring Butler County. The limestone in very dark in appearance, being a nearly black rock.

Locally, 100% 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 or Woods Run Limestone. These can be mistaken for Brush Creek and has been many times in the recent past in Western Pennsylvania. I am still not 100% sure I am looking at Brush Creek, but I don’t have any evidence against it yet. The Freeport Coal from historic measurements sits 120 feet below the layer where I find my limestone.

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, 1/256mm in size or smaller, become deposited by wind, water or ice. Sandstones are created much the same way. The exception is that they are made of much larger pieces of sediment, 1/16 – 2mm in size.

Limestone is formed mostly in clear, shallow, warm seas. It is made of at least 50% calcium carbonate (CaCO3) which is precipitated from the water. This mineral is in the form of Calcite which replaces the structures and features of the fossilized fauna within. It creates an interesting crystal pattern in some fossil specimens I find.

Where did Brush Creek Limestone come from?

Most local rocks occur at or around 305 million years in age. This was a time where rising sea levels would often flood the area. These are known as glacial-eustatic fluctuations in sea level. Western Pennsylvania is assumed to have been below the equator at that time. Using modern seas as an example, it is believed the water was warm, shallow and likely clear. This happened more than once it seems, as there are several laterally extensive limestone layers in the area. The Pine Creek and the Ames are two more examples of times when water rushed in and flooded the area.

However, these were not major quick flooding events. Over the course of tens to hundreds of thousands of years the water would come in and eventually the water would 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. Interestingly enough, this is known from fossil corals. These corals have a microscopic growth layer that accumulates each day throughout a year. Scientists counted these layers from fossil specimens and were able to provide estimates for length of year.

How do we know the ages?

Sedimentary rock is really difficult to date. Since it it 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 kids of rock. There are certain fundamental rules in geologic stratigraphy that are correct most of the time. Rocks further up are younger in age. There are some very unusual examples on Earth, however, where entire rock groups are flipped 90 or even 180 degrees.

When igneous rocks are dated, the half life of Potassium (K-AR dating) is measured in particular samples of rock. Since 50% of the potassium will have turned into Argon 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 does not appear to have been any volcanoes depositing rocks in the local later adjacent to limestone. There is a possible layer of volcanic ash in the Glenshaw Formation (the formation that contains the Brush Creek Limestone), but it has not been properly researched.

Ways to Correlate Strata

There is another way to correlate rock groups over long distances. When geologists started naming the local layers, it was easy to get confused early on. The Brush Creek and Pine Creek Limestones are quite similar to the casual observer. An attempt to identify and correlate layers by fossil species has been tried several times. It can be very successful, for example, there hasn’t been any Tainoceras found below the Ames and Woods Run Limestone. However, this doesn’t always work over long distances.

There is where conodonts come in. These fossil vertebrates are useful for identifying and correlating layers. They also evolved into different species quickly it seems. Their jaws fossilize easily.

LimestoneConodont Species
Amesl. simulator
Woods RunS. gracilis
NadineS. gracilis
Pine Creekl. confragus
Pine Creekl. cancellosus
Conodont index species across local Western PA Limestone (P. Heckel et el., 2011)

Examples 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 Limestone
Polished piece of Brush Creek Limestone. A horn coral species is seen in the top right of the specimen.
Brush Creek Limestone in situ. A tree has taken root above and is pushing it down.

Compared to the Ames Limestone

The Ames Limestone is a much lighter matrix, likely due to a less anoxic environment when the sea invaded the land. As the creatures living in the sea die or shed living biomass, it settles on the bottom of the sea. With more oxygen in the water, these decompose easier due to the oxygen. The Ames is a younger limestone, with a separation of approximately 2.3 million years of time between the two ancient seas.

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

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.


  • Heckel, P.H., Barrick, J.E., and Rosscoe, S.J., 2011, Conodont-based correlation of marine units in lower Conemaugh Group (Late Pennsylvanian) in Northern Appalachian Basin: Stratigraphy, v. 8, p. 253-269.
  • Hughes, H.H., 1933, Topographic and Geologic Atlas of Pennsylvania No. 36 Freeport Quadrangle, Pennsylvania Geological Survey Fourth Series