Cincinnati’s Geological Bedrock Is Famous

Geologists name lots of things for Cincinnati because our bedrock is unique and world-famous. Here’s the dirt on our world-renowned limestone, shale, and fossils.

The stage for Cincinnati’s formation was set a billion years ago when this area sat on the coast of what later became North America. That proto-continent slammed into another continent drifting in from the east, smooshing up a mountain range from Michigan to Alabama that geologists inexplicably call the Cincinnati Arch.

Illustration by Zachary Ghaderi

Those ancient mountains eroded over time and sank beneath a series of shallow seas teeming with marine life, some as familiar as coral and some extinct like the trilobites. The time period when our Upper Ordovician rocks and fossils were deposited, about 450 million years ago, has been named the Cincinnatian Epoch.

Geologists name lots of things for Cincinnati because our bedrock is unique and world-famous. Page through the major earth-science literature, and you’ll find references to fossil-rich rock known scientifically as the McMillan Formation or the Fairview Formation. The Edenian Stage of the Ordovician Period is named for Eden Park. Other rock layers commemorate Corryville, Madisonville, Mt. Auburn, Miamitown, and Mt. Hope Road in Price Hill.

As those primordial seas rose and fell, they left behind alternating layers of limestone and softer shale. Commuters navigating the I-71/75 “Cut in the Hill” in Northern Kentucky can view a dramatic example of these hillside “layer cake” deposits.

Cincinnati’s limestone is rock-solid and constitutes the many “fieldstone” walls that line our more venerable streets. Those stones, despite the name, did not come from any field—quarries atop Price Hill, Mt. Auburn, and Fairview churned out native building stone for a century or more.

Our shale, on the other hand, turns into a slippery, oleaginous muck when saturated with rainwater, which is why Cincinnati has the highest per-capita damage from landslides of any U.S. city. If your commute along Columbia Parkway has been detoured due to hillsides pouring onto the roadway, you can appreciate our wet shale.

Paleontologists, a contrary bunch, appreciate a good landslide because it exposes new specimens, and Cincinnati’s offerings are treasured around the world. No respectable natural history museum considers its collections complete without some representative samples of Cincinnati fossils.

Much of this reputation was built by paleontologists who got their start picking up a trilobite or brachiopod from a nearby creek and following that inspiration into a professional career. In his magnificent Rising from the Plains, John McPhee proclaimed, “Geologists tend to have been strongly influenced by the rocks among which they grew up. Cincinnati has produced an amazingly long list of American paleontologists—Cincinnati with profuse exceptional fossils in its Ordovician hills.”

Few American cities claim their very own official fossil, but Cincinnati does. Quarter-sized Isorophus cincinnatiensis is described by Brenda Hunda, curator of invertebrate paleontology at the Cincinnati Museum Center, as “an upside-down starfish on a dinner plate.” It was named in honor of our city in the 1850s by a German scientist. Cincinnatians will be able to better appreciate the abundance of our Paleozoic heritage when the Museum Center unveils its new Mission Ordovician: Cincinnati Under the Sea exhibition hall in 2023.

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