Glacial erratics: A rock like no other

  • Eric Aldrich’s wife, Adine, poses next to a large rock in Stoddard. Photo by Eric Aldrich

Published: 4/18/2016 6:00:58 PM

Like souvenirs from a long-past trip, some boulders in our woods are reminders of an incredible journey that happened over 12,000 years ago.

It was no journey any human would enjoy.

Under ice that became more than one mile thick, boulders were plucked from distant peaks and crushed and jostled through unimaginable pressure. Eventually, after thousands of years of violent jostling, the ice retreated.

In its wake, the glacier left lots of souvenirs, not just boulders. There are eskers, too, the result of inverted rivers flowing under the ice, leaving long, snaky ridges of sand and gravel. And it left striations, sharp lines in our granite left by rocks scraping against bedrock; you can see them pretty well on open places atop Mount Monadnock.

And there are kettle holes, drumlins, kames, moraines and many more reminders of New England’s last glacier.

But the glacial souvenirs I’m drawn to these days are the boulders.

They’re called glacial erratics. And they can be huge, the size of a car, a bus or even a house. What earns them the proud definition of erratics is that these boulders are different from the rock or soils under or around it. They were put there from somewhere else.

They look out of place. They sometimes sit precariously on another rock. They’re sometimes at the top of a hill, like lonely sentinels, leaving you to wonder how they got there. Who put ithem there? Was it chance or design?

How did they get here?

They beg for explanation, and back in the old days, before we really understood the Ice Age or its geologic processes, these big, out-of-place boulders inspired all sorts of theories. Some said they were caused by volcanoes blasting huge rocks. Some said it was the result of epic floods, like the one inspiring Noah to build an ark.

By the mid-1700s, Europeans were speculating the origin of odd, misplaced boulders throughout the Alps. In the 1800s, Louis Agassiz and Karl Friedrich Schimper had seen enough European glaciers to develop their own theory.

They suggested that Europe and other parts of the Northern Hemisphere had once been covered by glaciers, and those retreating ice sheets had left huge boulders and other features. Schimper even coined the term “ice age.”

The power of ice

If you’ve ever lived on a lake, you’ve seen what ice can do. If you haven’t removed your dock in the fall or if your dock isn’t exactly rugged, you’ll see that the ice has pushed and shoved the dock out of place, or crushed it altogether.

That’s just a few feet of ice over one winter on one little lake.

Now, imagine 75,000 years of ice, at one point more than a mile thick, right over our heads. We call it the Laurentide Ice Sheet, and at times it covered most of northern North America, stretching through Canada and ending around present-day New York City and Long Island.

This enormous ice sheet moved like a slow-motion, extra-wide freight train, crashing into anything in its path with incredible force and pressure. As it smashed southward into mountains, it moved and carved boulders off their peaks, embroiling them in its icy grip and moving them around for many miles. As the ice sheet melted and retreated, those boulders were deposited along the way. Hilltops and ridges sometimes pegged the boulders on the way, stopping them on or near the tops.

“Some of these boulders were carried great distances,” says Fred Rogers, a geology professor at Franklin Pierce University. “Some of the erratics in New England have been matched to bedrock in Canada, several hundred miles to the north.”

Rockin’ to the oldies

Some of our boulders – and those around Pisgah State Park – may come from a 400-million-year-old formation called Kinsman granite diorite, arising in Kinsman Notch of the western White Mountains, Rogers says. Some erratics in northern New Hampshire are believed to have come from the Canadian Shield formation that’s one to two billion years old.

It was a messy and random process, this whole retreating glacier thing. So random that you never know where the erratics may end up. Around here, glacial erratics are scattered all over the place, hidden or obvious in every town, and sometimes clumped in groups.

Some of Rogers’ favorite erratics are on Mount Watatic in Ashburnham, Massachusetts, where a trail goes right through a frost-split erratic. He likes another one in Dublin just off Route 137 near DelRossi’s Trattoria.

A few other notable glacial erratics include:

The Sarcophagus, shaped like an over-sized coffin on the Pumpelly Trail of Mount Monadnock.

Rollstone Boulder in Fitchburg, Massachusetts. This landmark rock possibly came from the summit of Mount Monadnock. It has a colorful history as well-meaning citizens sought to move the fragile stone from a gravel operation to a park.

Stoddard Rocks near the Stoddard-Washington line is well worth the hike. This group of mighty boulders sits atop a hill and was once a popular picnic spot.

Madison Boulder in Madison is New Hampshire’s proudest boulder and maybe its biggest. Estimated to weigh 5,000 tons, this whopper was designated a national natural landmark by the U.S. Department of the Interior as an excellent example of an erratic. If you’re in the neighborhood, check it out, but leave the spray paint behind.

Eric Aldrich writes from his home in Hancock.


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