You Know that Rockfall on the Palisades?

Scientists with the Lamont-Doherty Earth Observatory of Columbia University and staffers at the Palisades Interstate Park provide fascinating explanation and context for Saturday's rockfall.

Geology

First of all, "this was a rockfall, rather than a slide, in which a patch of the steep face of the cliffs either fell vertically or toppled and then broke up into large boulders as the debris hit the old scree pile below," said Colin Stark of Lamont-Doherty. 

Stark said the basic geology of the Palisades Cliffs is a dolerite sill or intrusion, which is to say an old injection of molten rock into the crust (~3 miles down or more) about 201 million years ago (at the boundary between the Triassic and the Jurassic).

This magma injection took a table-like form—flat, broad and relatively thin—once it froze into solid rock.  Erosion across New York and New Jersey has over time exposed this deep rock (the schist in Central Park is even deeper and older). Erosion during and since the last big ice age (about 25 thousand years ago) subsequently cut the main Hudson Valley and formed the Palisades Cliffs.

"The Cliffs are inherently unstable and slowing eroding westwards," Stark said in an email to Patch. "As such, it's not surprising we see occasional rockfalls at different points along them." 

Seismology

John Armbruster, a seismologist with Lamont-Doherty, said, "If you go online to our webpage and look at the seismograms obviously the rockslide stands as the largest amplitude on that day. There are some slower wiggles near it—that's a distant earthquake. The two are separate.

"As a seismologist I would note that it looks like a big deal, all those rocks falling—but this was pretty small. The explosion of Mount Saint Helens began with a big hunk of rock falling—a section of the side of the volcano—and that was recorded very far away." 

Armbruster is familiar with the view of  the Palisades from the east side of the Hudson—he is a graduate of Sleepy Hollow High School.

History

"Little rockslides happen all the time, of course, part of the natural process of erosion," said Eric Nelson, an educator with Palisades Interstate Park. "We have a good-sized rockslide, where you can see the results, probably every year, every other year. A really significant slide, where a lot of trees get knocked down or park roads are damaged, happens maybe every five to 10 years or so. The last really big one like that I can think of was in December 2005."

Here's the article about that 2005 rockfall.

Nelsen added, "This month is my 20th year working in the park, and I'm pretty sure the slide that happened Saturday is the biggest I've seen in that time. The biggest in park history was in July 1938, which was just massive, there's a photo here: http://www.njpalisades.org/SIL_1914.jpg"

Palisades Interstate Park

"The main thing is that we are truly grateful that no one was injured during the event," Nelsen said. "And we need to emphasize that the section of trail hit by the rockslide remains very unstable and will be closed until we are able to finish stabilizing it and making sure it is safe for hikers to use again."

Causes

"It's harder to explain precisely why this patch of cliff failed at the weekend," said Stark. "It may have been because of (1) long-term weathering and weakening of the joints and cracks that thread through the dolerite; (2) the effects of wintertime freezing and thawing, which progressively expands cracks by the repeated infiltration of water and its subsequent expansion on freezing (we had such a piddling winter that I wonder if this phenomenon happened at all this year); (3) some external shaking; (4) a change in the pattern of water flow underground."

A semi-serious suggestion on Patch that the failure might (karmically!) be related to the noise of a bunch of bikers going by Stark dubbed "very unlikely, but not impossible."  

He said another Patch commenter noted that a waterfall or at least seepage used to flow over the cliff face at this location. "And this flow might well be related to the failure," he said. "Yet another explanation might be thermal stresses induced by a rapid swing from a cool day to a hot sunny day, as it was on Sunday.  I would normally assume such stresses to be very weak, but in the absence of other clear mechanisms I wouldn't exclude it."

Learn more

Looking for more about the rockfall?

First, check out the Lamont-Doherty blog, which features a photo posted to Patch from alert kayaker Jack G. 

Second, look for the next edition of "Cliff Notes" on the park's website.

Third, go visit the park. The exits for State Line Lookout are well-marked on the Palisades Parkway both north- and southbound. 

"We happen to be working on an interpretive sign about the geology of the Palisades for the outdoor information kiosk at State Line Lookout, so the photos from across the river and seismic information from Lamont will really add a lot to that. It gives it all a real sense of immediacy," said Nelsen.