On December 14, 2016

Glacier-carved rocks, evidence of our Ice Age past

By Ned Swanberg

When I’m hiking, I like to watch for rock basins, sometimes as small as cupped hands, that appear along summits and ridgelines. These are “thin places.” When filled with water, these tiny quivering pools offer a deep plunge into time.
Basins run the gamut from Star Lake, a half-acre tarn beside New Hampshire’s Mount Madison, to dripping bonsai and moss gardens small enough to encircle with your arms on Mount Cardigan in Orange, N. H., to chalices of wind-churned puddles perched on the chin of Mount Mansfield in Vermont. Some of my favorites are “potholes,” the smooth rock bowls formed when pebbles and flowing water drill deep into the rock. Of course, most mountain ridges don’t have flowing water today.
There is a cascade that I visit often, just down the hill from where I live. In this spot the river has cut down through softer rock and created a swarm of potholes. I can easily soak my feet in some of these holes, while others suggest a cold and wild jacuzzi. Many of our New England cascades have these features, carved into active riverbeds, but largely formed thousands of years ago as the glaciers melted down at the end of the last Ice Age. Famous examples include the Basin in Franconia Notch, N. H., Sculptured Rocks in Groton, N. H., and Texas Falls in Hancock, Vt.
Aside from the well-known potholes, these water-carved rock features also appear surprisingly far from streams. In fact, a few appear on dry mountain cliffs. These remarkable features testify to the vast ice that covered all New England.
Walking up Mount Jefferson in New Hampshire, along the Caps Ridge Trail, most hikers tarry at a prominent outcrop to enjoy the thundering stream hundreds of feet below. But underfoot, and perilously close to the edge of the cliff, is a series of holes drilled into the rock. Some are dry. Others are vessels that hold rippling water, mineral colors, skylight and shadow.
Here on the side of the mountain, a still higher mountain of ice once stood. As it melted, ice and rock fragments plunged down a crevasse, bearing down at the very edge of the cliff. The glacial flow bored into the ledge with a furious slurry of abrasive stones. This powerful drill-set is called a “moulin,” or mill, by geologists.
In November, I walked up the Hedgehog Trail to Burnt Rock Mountain in Fayston, Vt. As I headed uphill, the foot bed of the trail eventually became a stream of ice, slush, and meltwater that scoured away the freshly fallen leaves.
The summit of the hill is a slab of dark gray schist studded with purple garnets and swirls of blue-white quartzite. These minerals are exceptionally hard. Their resistance to weather and erosion has allowed the summit to remain uplifted above the Mad River and Champlain valleys to the east and west. South of the summit and the steep ledges, just off the trail, I found what I was looking for: a mossy, fir-shadowed well.
The Burnt Rock Mountain pothole is only an arm’s reach across, and a bit deeper. Once drilled by immense power and abrasive stone, the pool has been tranquil for thousands of years. Now, the pool is refreshed by a trickle of water emerging from a rock crevice.
A hundred miles south of here, on the side of Vermont’s Mount Equinox, there’s evidence of another glacial moulin’s power. Glacial melt-water carried crushed schist off the mountain cap and drilled holes in soft white marble. Some of these marble chalices hold water and reflect the stars and clouds above.
Some do not. They are truly thin places: portals that pass entirely through the rock, to reveal the open air and valley below.
Ned Swanberg is a naturalist living in Montpelier Vermont. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine, and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: [email protected]. A book compilation of Outside Story articles is available at https://www.northernwoodlands.org.

Do you want to submit feedback to the editor?

Send Us An Email!

Related Posts

Pies, parades, and porch chats

July 2, 2025
“America is a tune. It must be sung together.”—Gerald Stanley lee The month of July is the height of summer, bringing a spirit of celebration to all of us. Our town of Killington may be small, but we know how to celebrate the 4th of July. We start early with the annual book sale at…

Inventing a better ski day: the innovations that drew crowds to Killington

July 2, 2025
By Karen D. Lorentz Editors’ Note: This is part of a series on the factors that enabled Killington to become the Beast of the East. Quotations are from author interviews in the 1980s for the book Killington, A Story of Mountains and Men. “We’ve got a million dollars that says you’ll learn to ski at…

‘Almost Heaven’

July 2, 2025
The stage was simple, designed to resemble a wooden board that resembled the siding of any barn, anywhere across America. It could have been the barn behind my house, or the one that my cousins have down in Georgia. It could have been a barn in Colorado or even West Virginia.  Nothing remarkable at all,…

Getting away from it all

July 2, 2025
My family and I went to the beach this past week. The temperatures were hot, and the weather was sunny, making for a classic seaside vacation. The house we rented was in the harbor of the town where we were visiting, so while we didn’t stare out at the ocean, we were able to sit…