What was NH like during the Ice Age? You can find clues at the Flume Gorge — and far beneath some local pond


New Hampshire Public Radio

Published: 09-04-2023 7:29 PM

It might be hard to imagine now, given the region’s lush trees and trails, that New Hampshire, and really all of New England, was at one point a giant glacier.

“Over the last 2.5 million years, we have had many different ice sheets come over this part of the country,” said Brenhin Keller, who teaches earth sciences at Dartmouth. “They would start from sort of around Hudson’s Bay in Canada and they would grow out from there.”

Keller specializes in studying the Earth’s continental crust, including how glaciers affect the planet’s surface. During the last Ice Age, about 26,000 years ago, he said the ice sheet covering New Hampshire stretched down to what is now known as Long Island, New York. If there was a period of really cold weather, ice sheets in North America could grow up to 3 miles deep.

To put it into perspective, Keller said that likely covered all but the tips of the White Mountains.

“For certainly a good amount of time, those might have been the only mountains, the only land above the ice sheet. In other times they were also glaciated,” Keller said. “The presidential range would have been sticking out above the ice most of the time. That would have been distinctive relative to any of the other states on either side.”

Conditions in New Hampshire during the last Ice Age were kind of like Antarctica, Keller said: Coldest and snowiest in the middle, with pools of water during warmer periods, or around the edges. Those water sources would have been the few spots where you could find wildlife.

There might have also been water underneath the glacial sheet covering New Hampshire, Keller said — and it’s possible it came from geothermal heat, from our state rock.

“Granites tend to have, relatively speaking, a lot of uranium and thorium and potassium, which are all radioactive and emit heat as they decay,” Keller explained.

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Ice ages tend to occur in fluctuating cycles, Keller explained. Scientists continue to study the effects of the last one — in New Hampshire and elsewhere — to make sense of the potential effects as the planet continues to warm.

Signs of the Ice Age in Flume Gorge

To find some evidence of the Ice Age, you can head to one of the White Mountains’ most popular tourist destinations: the Flume Gorge.

This naturally formed, narrow granite channel was forged around 200 million years ago, according to New Hampshire State Parks.

“The gorge was covered by glaciers during the Ice Age, but the ice sheet did not greatly change the surface,” the state says. “It partially filled the valley with glacial debris and removed soil and weathered rock from the vicinity. After the Ice Age, Flume Brook began to flow through the valley again.”

Today, the gorge is nestled in a forest, and there’s foliage growing everywhere. Walking along the roughly two-mile loop through the gorge, Conway granite walls tower above, reaching heights of up to 90 feet. About midway through, a waterfall breaks overhead, and a narrow wooden path zigzags across the channel.

According to Keller, sometimes glaciers greatly shape the landscape below, other times not as much. The key is in what’s happening at the bottom of a glacier. If there’s water underneath the glacier, he said, then it’s more likely to leave behind fjords or narrow bodies of water. If the glacier is frozen, he said, it doesn’t always make new landforms.

At Flume Gorge, attendant Ricky Aldrich-Holmes said you can see the remnants from the last glacier if you look closely at the walls of the rock.

“There’s a thing called glacial striae, or chatter marks,” he said. “Those are basically like cracks and grooves that the heavy ice puts over the rocks.”

Those cracks and grooves can indicate where the glaciers originated from, he said.

“That’s sort of our evidence to prove that these high 4,000-foot mountains were covered in ice at that point,” said Aldrich-Holmes, who recently graduated from the University of New Hampshire, where he studied geology.

Sometimes, glaciers carried rocks with them from other places. As ice expands or retreats, large boulders can get stuck in the ice. Eventually they become dislodged, leaving behind ”glacial erratics” that wouldn’t otherwise be found in that area. A few of these giant boulders can also be found at the Flume.

The current climate connection

Scientists study how glaciers shaped places like New Hampshire during the Ice Age in part because they want to know what happens when they melt.

Meredith Kelly, who teaches earth sciences at Dartmouth, is doing just that. Her research recently took her out on several New Hampshire lakes, looking for clues about how the temperature changed during and right after the previous ice sheet retreated. By taking samples from the bottom of the lake and studying what’s in them — pollen, insects and more — she tries to piece together a history of what the temperature, plants and other wildlife looked like over time.

During one trip to Occom Pond in Hanover, they examined sediment from roughly 8 to 9 meters below the lake floor, dating back to when the Laurentide Ice Sheet receded from the area about 13,000 to 14,000 years ago.

The National Science Foundation is helping to fund Kelly’s work in this area. Studying what happened when glaciers melted in the last Ice Age can help put today’s climate changes — like the melting of the Greenland ice sheet — into perspective.

“We want to improve our ability to say ‘How fast is it going to melt?’” Kelly said.

The last ice sheet, she said, didn’t all melt at once.

“It would like melt, melt, melt,” she said, “and then it would pause for a little bit.”

But there’s an important difference between the changes seen during the retreat of the last Ice Age and the changes happening to the planet today, Kelly said: Back then, humans were not putting greenhouse gasses into the atmosphere, accelerating the pace of global warming. In the past, the ice was melting from natural climate warming conditions.

“There’s now two major drivers of climate change,” Kelly said. “There’s humans and there’s the natural system and it’s not just one operating on its own. We need to understand the interactions between the two.”

These articles are being shared by partners in The Granite State News Collaborative. For more information visit collaborativenh.org.