In Columbus' Clintonville neighborhood, Elizabeth Heiser was working in her backyard when she made an unusual discovery. While removing some unwanted bushes, she found an almost perfectly sphere-shaped rock.
"We found this thing, which I immediately thought was a cannon ball," Heiser said, "I spent a lot of time trying to find out if there were stone cannonballs to find out that there were not. And then I wondered, what is it?"
Heiser happens to be a neighbor of mine, and I suggested she ask WOSU's Curious Cbus project about her mysterious rock.
It turns out that the grapefruit-sized sphere is called a concretion. They're millions of years old, and they're everywhere in Ohio.
To learn more, we contacted Erika Danielsen, a geologist with the Ohio Department of Natural Resources. Danielsen suggested that we meet at Shale Hollow Park about five miles north of Columbus.
The creek that cuts through the park winds between tall cliffs of red, gray and brown shale. Walking along the water, we immediately saw large round concretions sticking out of the cliff side and poking up from the creek bed. These concretions were much bigger than the one Heiser found in her backyard.
Danielsen explained that some of the biggest concretions can be found here in the Ohio Shale.
"We can find them anywhere from like centimeter size, really little, to like 9 feet," Danielsen said.
Understanding shale is key to understanding how concretions develop, she said.
"When shale forms, it’s originally mud that gets deposited at the bottom of an ocean" Danielsen said. "As that gets buried and pressed down by more and more sediment, eventually, all that mud will compress, all the water will be squeezed out of it, and it will be turned into the rock we see today."
This all happened back in the Devonian age, about 360 million years ago. It was a time before dinosaurs known as the “age of fish.” The sea that covered Ohio was swarming with life, and the fossil record indicates that a species of enormous armored fish called Dunkleosteus hunted for prey in the waters near the surface.
When one of these giants died, a part of its body such as a fleshy jaw bone might sink deep into the thick mud on the ocean floor. The decomposition of that organic material is how concretions begin.
"Bacteria and microorganisms will start breaking down organic matter around that jaw bone and that will change the chemistry in the mud and water around that jaw bone," Danielsen explained. "The change in chemistry allows for the carbonate minerals to start precipitating in that zone."
The chemical changes create a sort of bubble where other minerals can settle. In this case, it’s carbonate minerals similar to what makes up rocks like limestone and dolomite. After a few million years, those clay minerals in the mud have compressed into layers of shale and the carbonate mineral-filled bubbles have solidified into the sphere-shaped rocks we call concretions.
When Heiser learned that paleontologists sometimes find fossils inside concretions, she was curious about the chances her own might contain one. Danielsen said the probability is pretty low.
"There’s probably more concretions without fossils in the center than there are concretions with fossils, but you can never tell from the outside," Danielsen said.
Geologists study concretions to find clues about ocean chemistry and mineral deposits in the Devonian period. But there is still plenty to discover and debate about what they mean.
"There’s more questions around concretions than answers that they give," Danielsen said.
Concretions aren’t particularly valuable like other precious rocks or crystals, but they are a fascinating window into Ohio’s distant past - as well as a reminder that you never know what you might find in your own backyard.
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