|A section of the Palisades below State Line Lookout in Alpine, NJ, fell on May 12, depositing a mountain of new boulders along the Hudson River.
A 500-foot rock face came crashing down from the Palisades cliffs along the Hudson River in Alpine, N.J. on Saturday night, shaking the ground for more than half a minute and dumping a fresh layer of boulders over a 100-yard strip of parkland below State Line Lookout. The shaking was strong enough to be registered by a seismic station a mile and a half away, at Columbia University’s Lamont-Doherty Earth Observatory, but no one was injured.
The rocks fell on a shoreline trail called Giant Stairs, a massive boulder field created by several thousand years of accumulated rock slides. Eric Nelsen, an educator at Palisades Interstate Park, said Saturday’s rock fall is the biggest he has seen in his 20 years with the park service. “Looking down on it from the top of Lookout Point it’s a sheet of rocks plowed right into the river, with some chunks the size of school buses,” he said. An entire swath of trees—oak, black birch and paulownia—were swept into the river. In the next week, the park service will use heavy machinery to stabilize the boulders to make the trail safe again for hiking, he said.
|As magma solidifies, it forms cylindrical columns seen in this sample held by geologist Mark Anders. (Kim Martineau)
The steep Palisades cliffs formed during massive volcanic eruptions some 200 million years ago. When magma solidified, it formed columns of basalt, now exposed as steep cliffs on the western bank of the Hudson extending from Jersey City to Nyack, N.Y. Over millions of years, other rocks surrounding the basalt columns eroded, leaving sheer faces with long vertical cracks that make the Palisades susceptible to rock falls, especially during spring, as ice frozen into crevices thaws, loosening the outermost face. “The vertical cracks allow successive layers of columns to peel off,” said Mark Anders, a geologist at Lamont-Doherty. “As each column peels away, a new one is exposed, giving the cliff the good old Palisade wall look.”
The seismograph at Lamont-Doherty in Palisades, N.Y. was one of four stations in Lamont’s seismic network covering the northeastern U.S. to pick up the signal. The Palisades station recorded a stronger signal than stations at Fordham University in the Bronx, or in Basking Ridge and Ogdensburg, N.J., but even with its proximity to the slide, the Palisades station barely registered the equivalent of a magnitude 1 earthquake, said John Armbruster, a seismologist at Lamont-Doherty. The Palisades station has picked up other ground-shaking events in the New York metropolitan region unconnected to earthquakes, including the impacts on Sept. 11, 2001 of two hijacked planes flown into the World Trade Center as well as the subsequent collapse of both buildings.
Lamont-Doherty seismologist Bill Menke estimates that 10,000 tons of rock fell. Listen to the podcast of Menke’s May 16 interview with John Gambling on WOR710 News Talk Radio, and read his piece, Lessons from a Rock Fall on the Earth Institute's State of the Planet blog.
See the Pleasantville-Briar Cliff Manor Patch for additional photos.