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Ambridge Fossil Site


Guide to Fossil Fern Hunting near Pittsburgh, PA

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Ambridge Fossils

Fossil Hunting Guide to Road Cuts around the Pittsburgh Area

Coal Forest and marine environment
~ 299 - 300 Million Years Old
Upper Pennsylvanian, Westphalian D to Stephanian A
Conemaugh Group: Glenshaw Formation: Mahoning Shale and Brush Creek Marine Zone

Travel around Pittsburgh to find Fossil Ferns!

Fellow fossil hunters collecting at the Mahoning fossil fern layer near Ambridge, PA

Carboniferous Fern Fossils of Ambridge, Pennsylvania

A Calamites stem fragment from the Mahoning formation near Pittsburgh.

This is a large fossil plate of a bunch of Carboniferous plants. It looks like part of some kind of tree and some tree fern frond pieces. It shattered when it fell from the layer.

Geology and Paleontology of the Fossil sites near Pittsburgh, PA

North America 300 million years ago - The Carboniferous

In the United States, the Carboniferous period (360 -280 Million Years ago) is divided in the Mississippian (360 - 320) and Pennsylvanian (320 - 280) periods. During the middle Pennsylvanian period, the supercontinent Pangea, meaning "all land", had formed along with the Appalachian mountains, running north and south near the center of Pangea. The new Appalachian Mountains were some of the largest mountains of all time, and were probably as tall as the present day Himalayas. East of these Appalachian Mountains, which ended in central Pennsylvania, were large flood deltas, or flood plains. These large alluvial flood plains extended the entire way to the west coast of Pangea.

Image from the USGS (Public Domain) showing the United States in the Carboniferous Time period. Notice, the giant Appalachian mountains running along the east coast and the swamps covering much of Pennsylvania.

Carboniferous Coal Forests

Around this time, Pennsylvania was approximately located 5 degrees south of the equator, in a tropical rain forest type climate with very little seasonal fluctuation. Also, at this time, the Earth had much higher concentrations of carbon dioxide in the atmosphere. This was very conducive to the growth of plants. Hence, in these flood deltas, large, swampy forests grew. However, the plants in these forests were far different than plants of the present day.

Flowering plants, or deciduous plants of any kind had not yet evolved. Instead, there were very large, simple, fernlike plants, such as Calamities (a giant horse tail), Lycopods (which grew up to 100 feet) seed ferns and herbaceous ferns (that grew up to 50 feet). However, at the end of the Pennsylvanian, most of these strange plants became extinct, and deciduous plants eventually evolved. Much of these forested areas became buried and now create the carboniferous coal beds that are mined today throughout the eastern United States and Europe. Because of this, these swampy forests are often called coal swamps and coal forests.

This is figure 3 from (Gastaldo, et al. 2004). It shows the reconstruction (based on 17 samplings) of an actual fossilized coal forest preserved above the Blue Creek Coal in Alabama.

In these forests, new insect life also flourished, such as dragonflies, mayflies, millipedes, scorpions, and spiders. However, these were not normal insects, they were giant man eating creatures. Well, the may not have been man eating, however some dragonflies had wingspans of 2.5 feet, cockroaches were a whopping 4 inches, and flies needed extra large fly swatters. Unfortunately, these fragile insects did not readily survive fossilization in the coal swamps, and only rarely can be found as fossils.

A unique feature of this period was the development of the amniotic egg for reproduction. This basically means animals could now lay eggs on land. So, very primitive reptiles, such as Hylonomus and Anthracosaurs evolved. Also, around this time, large amphibians existed. Fossilized jaw fragments from these amphibians can occasionally be found in outcrops around the Ambridge area.

Transgression and Regression Cycles

During the Carboniferous, similar to today, the Earth experienced Ice Age Glaciation events. When Glaciers advanced in Pangea, the sea levels would drop. During interglacial periods (similar to today), sea levels would rise. The sea level rise and fall, called transgression and regression cycles continued for millions of years during the carboniferous.

Every time the Panthalassic ocean (which was almost like the Pacific Ocean, but on a larger scale) on the west coast of Pangea flooded the lowland deltas, a marine environment was created where there once was a tropical coal forest. The forest was buried, compressed, and eventually turned into a coal layer.

These sea level fluctuations can be seen in the rock units as a layering pattern of coal and marine limestone. This pattern can clearly be seen in the Stratigraphic Column below in the next section.

Diagram of a transgression and regression cycle - Periodic glaciation events in Pangea would cause a constant sea level fluctuation. This occurs today. During the Ice Age, over 10,000 years ago, the sea levels were very low. Today, we are in an interglacial event, and the sea levels are high.

Fossils from the Ambridge Road Cut

The roadcut next to the Ambridge Woodlawn bridge contains part of a transgression / regression cycle. The first level (at road level) has the preserved remains of one of the Pennsylvanian coal swamps. They are in shale from the Mahoning coal. Above the remains of this coal swamp, is a marine environment, the Brush Creek Marine Zone.

Mahoning Shale - Coal Forest Fossils

The Brush Creek Marine Zone is composed of black, oily shales and limestones. These shales contain numerous gastropods, and straight shelled nautaloids. Ancient fish also lived here, like placoderms and primitive forms of sharks and occasionally their small teeth are found around this area (although I know of no one actually finding one at this particular site).

Stratigraphic Column of the Conemaugh group near Pittsburgh, PA

The diagram below shows a general stratigraphic column of the Conemaugh group. The lower half is the Glenshaw Formation. The roadcuts around Ambridge contain the lower part of he Glenshaw Formation, from the Brush Creek Limestone to the Mahoning Limestone.

The BLUE bars are limestones formed in marine environments, the BLACK bars are coal beds formed from coal forests, and the YELLOW sections are channel sandstones when sand filled in river channels.

By glancing at the colors, one can clearly see the marine transgression / regression cycles that make up this formation.
This is a generalized column, in reality there are many other rock layers, including the red beds and paleosols.

Stratigraphy of the Conemaugh Group. The Ambridge site exposes the Brush Creek Limestone down to the Mahoning Limestone. This diagram is from The University of Pittsburgh Department of Geology and Planetary Science.

Fossil Collecting Location near Pittsburgh


This road cut and many others in the Pittsburgh area are on/near busy roads. Please use common sense when collecting at road cuts. Also, make sure you have permission first before collecting on any private property.

Do not to fossil collect along a busy road or a road with a narrow shoulder.
Do not park your car along a busy road or a road with a narrow shoulder.
Do not fossil collect at a tall road cut, or collect at the base of a tall road cut, the rocks fall all the time!

The author and host of this site bear no responsibility for injury sustained by those who visit the sites listed herein.

Google Map of the Ambridge / Aliquippa Roadcut

The road cut is located on the side of Route 51 across from the Ambridge Woodlawn Bridge on the west side (Aliquippa side) of the Ohio river. It is clearly visible when crossing the bridge.

Most of the roadcut is not accessible because the cut is very high and therefore too dangerous to collect at. Also there is no parking space at the actual road cut.

If you want to fossil collect at this location, you need to go to either the southernmost section or the northernmost section. This is where the Jersey barriers end and the shoulder widens. At these places, the cliff is not nearly as high or steep, and there are places for safe parking about 1/8 - 1/4 mile down the road from the end of the cut on both the north and south ends. Don't park on the shoulder, as this is dangerous.

Other Locations

Pittsburgh is full of roadcuts. Most of these roadcuts around Pittsburgh contain part of the Conemaugh Group. This means many of these cuts have either Carboniferous plant or marine fossils. Some of the cuts contain Permian fossils as well.

The location listed above is just one of many. Search around and explore. You can find many safe locations to collect fossils all around the greater Pittsburgh area.

Recommended Equipment for splitting shale and mudstones

INCLY Amazon Store: Rock Hounding Gear

INCLY provides budget-friendly rock hounding gear, ranging from the widely favored (4.6/5 stars) 15-piece Geology kit featured in the image to compact 4 and 5-piece sets. These tools are ideal for fossil and mineral collecting, particularly in harder matrixes and shale.

Rock Pick - 28 oz Geological Hammer with Pointed Tip & Shock Reduction Grip - 11.4 Inch

I bought this as a replacement rock hammer. The color makes it stick out so you don't lose it in the field! It's a little heavier than your average hammer, making it easier to break rock. It also has a nice grip and for under $20, it's a great deal!

TOOLEAGUE Heavy Duty Masonry Chisel,8 inches Brick Chisel with Hand Protection(1Pc)
Albert B Dickas, 2018

I learned years ago to only use chisels with hand guards! The bright color makes also makes it hard to lose when you set it down. This is a nice wide chisel ideal for splitting thin shale.

Pry Bar Long prybars, a couple feet in length, are convienant for separating large slabs.

Safety Glasses:

This should go without saying... Anytime you hit a rock with ANYTHING, you should be wearing protective glasses. One tiny rock fragment can cause blindness if eye protection isn't worn!

Glue / Aluminum / Insect Repellent:

Other collecting items should include aluminum to wrap the fragil fossils in, glue for quick onsite repairs, and insect repellent. There is allot of vegetation at the road cuts and ticks can be a problem.

Common Sense:

Use common sense while in the field. Road cuts, by definition are along the side of a road. It's better to park in a parking area and walk the extra distance than parking along the side of the road.
Don't collect right next to a busy road. Make sure there's plenty of room between the road cut and the road!
Also, road cuts are large cliffs. Stuff falls all the time. Stay away from steep or unstable looking cuts. Don't climb the cuts, and stay away from the base of the cuts. Collecting at the tullus slopes (rocks that already fell) usually provides more than enough rock to look through.

Don't Overpack:

I often see collectors who look like they are hiking the Appalachian trail for 6 months. Don't overdo it. You want to have fun while collecting. You don't want to have to carry 30 pounds of MRE's, 2 tents, a kitchen table, 3 changes of clothes, a rubber ducky, etc...
Idealy, you want to pack light and come home heavy with fossils.

View a Sample of Fossils Found at the Ambridge area:

If you plan on collecting plant or marine fossils from around Pittsburgh, this section will aid you in fossil identification.

Since there are two distinct type of fossils found in the Glenshaw Formation here, it is broken into two pages, the Fossil Plants and the Marine Zone fossils.

Plant Fossil Identification Page for the Carboniferous Fern Forests.

Marine Fossil Identification Page for the Marine Zones.

Recommended Books for Fossil Plants:

A Guide to Pennsylvanian (Carboniferous) Age Plant Fossils of Southwest Virginia
Thomas F. McLoughlin, 2019

Although it's for Virginia, these are the same fossils for the Carboniferous of PA, MD, and WV. It was one giant forest that ran down the ancient delta. This book is full of illustrations, 280 to be exact, many are in color. The illustrations make identifying most carboniferous plants a simple task. If you collect Carboniferous plants, this well organized book will serve as your identification guide.

101 American Fossil Sites You've Gotta See
Albert B Dickas, 2018

This is a great updated fossil sites book with at least one fossil site in each state. Each site is broken into 2 pages. One has detailed information, such as directions, GPS coordinates, formation information, etc... The other is dedicated to images of the site and the fossils found there. It also gives information on fossil 'viewing' sites such as dinosaur trackways, museums, and active excavations.
Plus, my fossil photos are peppered throughout this book!

References / Works Cited

Gastaldo, Robert A.; Stevanovic-Walls, Ivana M.; Ware, William N.; and Greb, Stephen F. (2004) Community heterogeneity of Early Pennsylvanian peat mires. Geology 32, 693-696. (doi:10.1130/G20515.1)

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