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.