Spherical wonders: Geologists have a few theories about how concretions formed

The ravine at Shale Hollow Preserve is a gray, red, orange and brown stack of shale that resembles a pile of papers.

The wall, which borders Big Run Creek, mostly is uniform — row after row of flaky rocks. But there is a spot near the footpath, about 6 feet up, that is anything but ordinary. There, embedded in the shale, is a perfect sphere of rock the size of a beach ball.

These mysterious, rocky spheres are located across the state. The smaller ones often resemble cannonballs; the bigger ones, many of which have fallen to the ground and flattened, resemble small hot tubs or bagels.

They’re called carbonate concretions, and they are among Ohio’s greatest geologic mysteries. They are made of limestone or dolomite and form inside layers of shale, pushing the shale away as they grow.

“It’s like a marble in a book,” said Rich Niccum, education services manager for Preservation Parks of Delaware County, which manages Shale Hollow. “If you take a marble and press it into the book, the pages go up and around it.”

Scientists aren’t sure how the concretions formed, though they do have some theories.

Carbonate concretions are found mainly in Ohio shale, a layer of rocks formed after the Devonian Period around 360 million years ago.

Devonian shale cuts through central Ohio and runs along Lake Erie between Sandusky and Ashtabula. There are Devonian rocks near Bellefontaine in Logan County and some in the northwestern part of the state, too.

During the Devonian Period, Ohio was covered by a shallow inland sea inhabited by small, bony fish; bacteria; and some plants. When those things died, their remains sank to the bottom of the sea and, over time, were compressed to make shale, a sedimentary rock.

At the end of the Devonian Period, there was a mass extinction — one of five great extinctions that scientists say occurred during Earth’s history. Some of the life that became extinct, scientists say, could have been the seed for the concretions we now see.

If that is true, concretions could have formed the way oysters turn grains of sand into pearls, said Eric Mumper, an Ohio State University doctoral student who has studied concretions.

Scientists have found fossils in the cores of some concretions, lending credence to that theory. But not all concretions have fossils at their hearts, and not all fossils became concretions.

Another theory: Concretions might have formed when water flowed beneath the floor of the inland sea, bringing new minerals to the shale and soil.

Concretions are made up of varying materials. Their cores are generally soft rock, which can wash away over time. Their shells are harder.

“Geologists liken it to a Tootsie Roll Pop — you’ve got that hard outside and soft inside,” Niccum said.

Concretions are found elsewhere, especially out West.

In central Ohio, they can be found at Highbanks Metro Park in Lewis Center and Glen Echo Park in the Clintonville area.

“You can find them other places, but you have to go to Utah or Colorado to see them,” Mumper said. “They’re already here in our backyard, and they are pretty neat-looking.”


Source: The Columbus Dispatch