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Don’t Take Me for Granite

By: Casey Smith, Preservation Parks naturalist

Do you have a giant rock in your yard? If you don’t, I’m sure you’ve seen them in others. Many of those behemoth boulders are granite, which does not naturally occur here in Ohio. So how did these rocks get here?

Around 2 million years ago, the Earth’s climate began to cool as it entered its most recent Ice Age. And yes, we are still in the Ice Age, just in a warm period.  The last southern advance of the Ice Age (approximately 70,000 years ago) is called the Wisconsinan glaciation and it is the one that had the most lasting effects on Ohio’s landscape. As snow began to accumulate in Canada the weight of it caused the snow to compact, spread out and move southward. As the ice moved, it collected everything in its path including trees and rocks, and dragged and pushed debris along, scraping and gouging the ground as it moved.
The glacial grooves found on Kelly’s Island were created by this scraping. The ice eventually reached central Ohio approximately 18,000 years ago, and was estimated to be 800-1000 feet thick (imagine the height of the Empire State Building). For comparison, ice that covered what is now the Great Lakes was more than a mile thick in places. Northwestern Ohio is flat as far as the eye can see because glaciers plowed everything out of their way and southeastern Ohio still has rolling hills because glaciers didn’t make it that far south.

About 14,000 years ago, the climate began to warm and the ice began to melt. The retreating ice dropped everything it had collected, leaving behind tons of rocks and other debris. The rocks left dotting the landscape, some the size of small cars, are called glacial erratics. Some erratics are visible on the surface but many remain below the surface waiting to be discovered. 

Shale Hollow, Hogback Ridge, Deer Haven and Char-Mar Ridge Parks are great places to look for glacial erratics as well as see the effects the last Ice Age had on Ohio’s landscape. Go check it out!

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