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.
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 farther away, at Fordham University in the Bronx, and Basking Ridge and Ogdensburg, N.J., but even so, 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.