hanging boulder falls polar caves NH on Sunday, when an 80-ton rock that had hovered over a trail at Polar Caves Park gave way after 104 years in place. Polar Caves Park announced the fall on Monday and said no one was injured.
The park’s management wrote, “We are incredibly grateful no one was injured in the incident.” A trail sign said the boulder weighed 80 tons, was cracked all the way through and rested on 3 contact points in the back, with 75 percent of its weight hanging over hikers’ heads.
Joe Licciardi on glacier placement
Joe Licciardi, a professor of glacial geology at the University of New Hampshire, said the boulder was deposited in its hanging position by a glacier thousands of years ago. He also said the park’s caves are spaces between enormous granite boulders that were at one point put there by glaciers.
That geology left the rock suspended above a stretch of trail long enough to become known as the Mysterious Hanging Boulder. It sat there through the full 104 years of the park’s existence before falling on the last day of the season.
Jan Earle on erosion
Jan Earle, a geologist who has worked with the U.S. Geological Survey and state parks across the country, said the rock had been balanced in place with some of its weight resting on its base and some hanging over the trail. “It was a balanced rock essentially, so if more of its weight was hanging over the edge than sitting on its base, then it was just a matter of time until gravity and erosion of its base took over, and it tipped over,” she said.
She compared the long process to aging metal: “It's just been sitting there for a long time, and it's sort of like a old car rusting and then finally falling apart.” Licciardi added that water freezing and thawing could have made the boulder vulnerable to erosion, and that water expands by roughly 9 percent when it turns to ice.
Trail damage at Polar Caves Park
A video posted online showed a railing for the trail’s wooden walkway smashed to pieces, while access to the rest of the trail was cut off by the boulder. The rock now sits upright on its side, blocking the route where visitors once walked beneath it.
The park said a safety assessment will be conducted during the offseason. Whether the boulder will be removed, left in place or used in future trail repairs has not been answered, leaving the park with a damaged walkway and a trail that cannot fully open until that review is done.
