Here’s a word lesson. Erratic: to lack a fixed course or uniformity; to wander or deviate from the ordinary. Now, let us apply that to Ohio’s geology, which is anything but ordinary.
The largest stones in Ohio aren’t from here. They are erratics — granite and basalt migrants from the north, boulders that were picked up in Canada and strewn about ... erratically — seemingly without any pattern or uniformity. How did they get here? Well, some background first.
What is past is prologue. In other words, that which has happened before our present time sets the context of the current age. And so it is with Ohio’s geologic past: a pageant of events over protracted bygone eras that go back an inconceivably long time, so long you can hardly wrap your mind around it. And yet here it is, right at our feet (mostly).
This photo of a glacial erratic boulder near Stratford was published in the Ohio Geological Survey’s Geology of Delaware County in 1926.
If you need a reminder that the world is held together by stone, then consider Ohio’s basement. You cannot see it except in the long strands of bored-out cores that have been extracted from hundreds of feet below the earth’s surface. The basement is composed of sedimentary rock, settled and compressed at the bottom of what was once a deep ocean. That bedrock dates to about 570 million years ago, geologists say. It’s called Cambrian period stone, named after Cambria (which is now called Wales), where the stone type was first described and given a name around 1835.
Above the Cambrian, right up to the ground you walk on in Ohio’s central, western, and southwestern parts, lie rocks from the Devonian and Silurian periods. Those names, too, come from across The Pond: the town of Devonshire, England, and the Silurians, an ancient Celtic tribe. Those rock formations, a mere 350 million years old, were the first to entomb fossilized shells and other organisms from that ancient sea, which explains how the official state fossil is of a marine species discovered near Dayton.
More recently (though still in the far distant past), Ohio endured the Pleistocene epoch, a cold and crushing ice age that started about 2.6 million years ago. The frigid climes brought glaciers down from Canada, covering two-thirds of Ohio with ice a mile thick. The glaciers gathered up rocks and anything else in their path, grinding away that old sedimentary stone as they crawled slowly southward. Only the southeast part of the state, the Appalachian Piedmont, was spared the icy erosion.
And then, about 12,000 years ago, a warming trend. As the massive ice sheets retreated back to the north (where they remain today), they dropped their rocky contents out of the melting ice, hundreds of miles and thousands of years from where they came. Typically, erratic stones lack sharp edges, having been subjected to tumbling, grinding, and polishing by the slow but abrasive contact with ice and the fine soils that were also shoved along by the glaciers.
You can find granite cobbles and boulders individually or clustered in small groups or bands throughout glaciated Ohio, but it is at the point of the glaciers’ greatest advance, roughly in a line from what is now Clermont County northeastward toward Columbiana County, where they are most plentiful. They are strewn erratically in creek beds and on farm fields, or revealed from just below the surface amid piles of earth excavated for new construction.
There are a few notable erratic stones around the state, not the least of which is the Sunbury boulder, which sits on what is now private property in Delaware County. It is a massive 22 feet long and 72 feet around at its base. The part of it that’s visible likely weighs 200 tons — though no one knows for sure how much more of it is buried.
The stones are of little economic value in and of themselves, though they do possess ornamental value as landscape specimens — if you can move them — and some erratics are large and prominent enough to draw in a tourist or two. More often, they’re likely to be a nuisance to agriculture and engineering, damaging farm implements and requiring heavy machinery to get them out of the way.
But to a geologist, erratic stones possess an intrinsic value. They provide maps to the past, revealing how the thick ice flowed, where they originated, and how long ago the ice dropped them.
So erratic glacial stones, in essence, are monuments — both to the deep, cold past and to the here and now. Each impressive rock is a tombstone that marks the exact spot where the glacier that carried it across the continent gave up the last of its energy and could carry it no farther.
