The Ames Limestone (Andrews, 1873) is the boundary and limestone stratum between the Casselman and Glenshaw Formations within the Conemaugh Group. This limestone can be found throughout Western Pennsylvanian, Eastern Ohio, and northwestern West Virginia. It is a laterally extensive stratum that contains what many call the most fossiliferous in Western Pennsylvania. Being limestone, the fossils within are mainly marine. An exception to this would be a land-based creature or plant falling into the sea and being buried, which is rare.

Rock layers of Parks Township Simplified stratigraphic column, youngest at the top: Monongahela Group, Casselman Formation, Glenshaw Formation with the Ames, Woods Run, Pine Creek and Brush Creek limestones, Allegheny Formation, and Pottsville Formation. Not to scale. ▲ younger Monongahela Group Pittsburgh coal Casselman Formation Casselman Fm. ◀ You are here Glenshaw Formation Glenshaw Fm. ◀ You are here Woods Run limestone Pine Creek Limestone Pine Creek Limestone ◀ here Brush Creek Limestone Brush Creek Limestone ◀ here Ames Limestone Ames Limestone ◀ here Upper Freeport coal Allegheny Formation Allegheny Fm. ◀ You are here Brookville coal Pottsville Fm. Conemaugh Group Late Pennsylvanian Middle Pennsylvanian ▼ older · not to scale
Rock layers of Parks Township, simplified and not to scale. Select a layer to read more.

Geologic Age

The Upper Freeport coal bed is the lower boundary of the Glenshaw Formation, while the Ames Limestone is the upper boundary. The Ames marks the boundary between the younger Gzhelian, the last stage in the Upper Pennsylvanian, and the older Kasimovian. While many older papers refer to names such as the Missourian or Virgilian Stages, the International Commission on Stratigraphy uses standard global names.

Global StageAgeTime Represented
Gzhelian4.8 MY298.9 – 303.7 MYA
Kasimovian3.3 MY303.7 – 307.0 MYA
Global geologic time stages and their approximate ages.

The map below shows Ames as the yellow-to-orange merge point. Published in 1980, this map is by the Commonwealth of Pennsylvania Department of Environmental Resources. One interesting detail about this map is that the freeway portion of PA 28 terminates shortly after passing the Freeport area. This shows the map’s age; it was published about 40 years ago.

Map showing the Casselman (orange), Glenshaw (yellow), Allegheny (green), and Monongahela Group (Tan) formations.
Key for Map
The key to the above map

The Ames preserves fossils dating back to the Carboniferous Period. More specifically, the late Pennsylvanian.

Here is a map of the Casselman Formation embedded on Google Maps.

In a 1933 report, H.H. Hughes noted that the Ames Limestone was nearly 30 inches thick near Perrysville, which I believe was in Bell Township, southeast of Vandergrift. In the Plum-Lower Burrell area, it was 16-35 inches thick and was utterly absent at two drill sites. Fragments found in the Northern part of the Freeport Quadrangle, South Buffalo Township, were 3 to 4 inches thick.

Compared to the Brush Creek Limestone

The Brush Creek Limestone has a much darker matrix, likely due to a more anoxic environment during the sea’s invasion of the land. A lack of oxygen means more of the biomass from the living creatures above doesn’t break down at the bottom of the sea. The Brush Creek is an older limestone deposit, separating the two ancient seas by approximately 2.3 million years.

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.

Exploring Ames Limestone

The first place I saw the limestone up close was in Frick Park. While there are several locations where I can find it, I have yet to find one with exposed rocks. I’ve done online searches for Ames Limestone and have often landed on the Sedimentation in Western Pennsylvania page, an old Pitt Geology web resource.

Following their directions, I quickly found two sets of outcrops. As mentioned, I could not find the hard ledge in the stream bed. However, there appears to be significant erosion in the stream bed, and several blocks of sandstone or limestone are present along the creek.

I did not bring a hammer or chisel. Quickly, I learned that you are not allowed to work on rocks in Frick Park. However, it was nice to finally see it up close.

Ames Limestone in Frick Park
Ames Limestone as Ledge
Ames Limestone in Frick Park
Ames Limestone Showing Fossils

The Ames Limestone in Northern Limits

In Penfield, PA, the farthest Northern mapped portion of the Ames exists a bit past DuBois, PA. This is over 60 miles from Parks Township.

While the Casselman layer may be eroded, it does not necessarily mean the Ames Limestone is gone. The stone remains long after the silty base layer has been eroded further. Float limestone can still be found in local streams after the more erodible layers have been washed away.

Ames Limestone along Route 28.

There are several places where the Ames crops out along Pennsylvania Route 28. During the road construction, massive amounts of the landscape were removed, exposing many hidden rocks. It can be seen more or less along stretches of the route from Etna to Tarentum.

Ames Limestone along PA 28 North
Ames Limestone along PA 28 North
Ames Limestone is near PA 28 North, just past the Pittsburgh Mills exit.

More Information about the Ames

References

Cross-section of Parks Township

Cross-section of the rock layers under Parks Township Simplified geologic cross-section about two and a half miles long, after J. A. Harper (2019). From the bottom up: Pottsville Formation, Brookville coal, Allegheny Formation, Upper Freeport coal, Glenshaw Formation with the Brush Creek, Pine Creek, Woods Run and Ames limestones, and the Casselman Formation, which survives only as caps on the highest hilltops. Vertical scale exaggerated about five times. Casselman Formation Glenshaw Formation Allegheny Formation Pine Creek Limestone Brush Creek Limestone Ames Limestone Glenshaw Formation Allegheny Formation Pottsville Formation Casselman Formation Ames Limestone Woods Run limestone Pine Creek Limestone Brush Creek Limestone Upper Freeport coal Brookville coal Kiskiminetas River 1,000 ft 1,200 ft 01 mile After J. A. Harper (2019) · vertical scale exaggerated about 5×
Rock layers beneath Parks Township along a line about two and a half miles long, after J. A. Harper (2019). The Brush Creek, Pine Creek and Woods Run limestones are drawn at their approximate positions within the Glenshaw. Select a layer to read more.