Erratically Speaking

If you travel through any rural area in northwestern Pennsylvania, you will invariably see large boulders dotting the landscape. Most often, they have become lawn ornaments of a sort in homeowners’ yards, marking the entrances to driveways or delineating property lines. These boulders vary in size all the way up to as large as a car and weigh several tons.

However, if you venture south toward Pittsburgh, by the time you get near Butler County, they become very scarce, and then as you head on farther, there aren’t any more of them to be seen at all. If you’re curious as to why they are here but not there, keep reading!
Here in Erie and Crawford counties, they are so common that often we hardly notice them, having seen so many of them over the years. While we might not give them a second glance, they have quite an amazing history to be told.

It’s hard to imagine that as recently as just 12,000 years ago, northwestern PA was covered by a glacier, the Laurentide Ice Sheet, upwards of a mile thick. The glacier formed in northern Canada and, over the course of 100,000 years, relentlessly flowed south, reaching northwestern PA about 20,000 years ago. As it flowed through Canada, it scoured the landscape. Nothing could stand up to its advance, with bedrock being torn apart by the ice and the pieces carried along with the flow.

The process of formation of glacial erratics. A. Glacial erosion entrains a boulder of the bedrock. B. Continued glacier flow transports the boulder. C. As the glacier retreats, the boulder is deposited on a different type of bedrock, forming a glacial erratic. (Image and Caption:

While being tossed and turned by the glacier as it continued to flow south, these pieces of bedrock became rounded and polished, still irregular in their shape but with distinctly smoothed edges. As the glacier flowed across the present-day Great Lakes basin and into northwestern PA, these pieces of Canadian bedrock had no choice but to go along for the ride.

The glacier continued to flow south as far as northern Butler County, but about 12,000 years ago, the glacier began to retreat. By 8,000 years ago, northwestern PA was liberated from the ice age. However, glaciers don’t really “retreat” in the sense of flowing backward. Instead, they just melted in place, and as they did, the pieces of Canadian bedrock were left behind, everywhere dotting the landscape. Their amazing free ride had come to an end.

A moss-covered glacial erratic is a silent sentinel below Howard Falls.

Northwestern PA is covered principally in Devonian shale, deposited in layers about 400 million years ago when this area was a shallow inland sea. The Canadian bedrock carried here by the glacier is distinctly different geologically. They just don’t seem to naturally belong here.

These glacially deposited boulders are called “Glacial Erratic.” They take their name from the Latin word errare, meaning “to wander .”And wander they did, having been carried hundreds of miles from central Canada during their incredible journey.
Next time you travel around rural northwestern PA, watch for Glacial Erratic. You will see more than you can count. We have plenty of them here at Howard Falls, too. Count on it!


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