Saturday, June 4, 2011

Big landslide on Little Porter Mountain

New York State's largest landslide is slowly dismembering the Machold family's home.  Covering 82 acres of a steep, wooded slope on the western flank of Keene Valley, it isn't exactly what you might expect if you have rockfalls or avalanches in mind; it moves only a few inches per day.  But that doesn't mean it's any less destructive to structures sitting on it or anything else, trees included, that lies in its ponderous path.  Just a few months ago, the Machold house was valued at $600,000; now it's worthless. So are several other residences nearby which are also being abandoned.

The Machold home, with the ground slipping off to the lower right.
 
Tree roots stretching as the forest floor pulls apart.
A tree trunk splits as the top of the slide drops away downhill.

NYS Museum geologist Andrew Kozlowski gave a presentation about the new slide at Keene Central School a few days ago, partly as a public service to the kids who live in the immediate vicinity of this thing, but also to offer a forum to curious citizens and the press.  Here are some gleanings.

This spring's combination of copious snowmelt and wet weather was the primary trigger for the "rotational slump" on Little Porter Mountain, perhaps in part by adding weight to the sloping blanket of loose soils and debris, but most likely by mobilizing the underlying sediments.  Nobody knows exactly what lies between the surface and bedrock there, but Kozlowski guesses that it's crumbly sand in old beach deposits left over from the end of the last ice age when the valley was a deep glacial lake.  When water soaks into the spaces between sand grains, they can lose their grip on each other and roll around like tiny ball bearings under the tug of gravity.

As the slide draws this boulder aside, it reveals the underlying groundwater that may be setting the slope in motion.

Nobody knows yet how far the slide will move or when it will stop. But now that experts are looking closely at it we find that it's actually not as unusual as one might think.  Step-like patterns in the surface contours tell of earlier flows that broke the terrain into blocks and sent them drifting downhill like rafts on a sluggish flood.  Judging from the century-scale age of trees growing on it (which also hid the evidence of instability from casual view until now), the slope may have been moving off and on since the proglacial lake drained away thousands of years ago.


The foot of the slide rolling through the woods downslope like a slow-motion tsunami...
... rolling over any trees in its path.

Many of us in the audience that day wondered what this means for the rest of the rugged Adirondack landscape.  Are other sites at risk?  "Yes," was the short answer we got, and I imagined realtors and homeowners all over the Park drawing a sharp breath.  Nobody's been watching for this sort of thing as pricy houses are built on high slopes with nice views, and it will take detailed region-wide mapping to reveal the trouble spots.  Kozlowski suggests that he best tool for the job would be an aerial survey done with LiDAR (Light Detection And Ranging), which apparently costs roughly $150K per county.   But at this early stage in the story, it's anybody's guess as to whether it will be carried out or who's going to pay for it.

The residences now perched along the upper rim of the Porter slide - or riding it downhill - were part of Adrian's Acres, a collection of high-end, thoughtfully designed properties developed by Adrian Edmonds who passed away recently at age 96.  I met him at his home in Keene Valley several years ago with another topic in mind; I wanted to learn about the old days when his family originally settled next to one of the Cascade Lakes, back two centuries ago when people around here called it "Edmonds Pond."  Though I came to his door as a nosy stranger, he was very gracious, warm-hearted, and generous with his time and knowledge despite the recent loss of his wife.  Now I can't help feeling grateful that he didn't have to watch this happen.  He seemed to be very proud of Adrian's Acres, and I suspect this spring's sudden destruction might have broken his heart.

Adrian Edmonds, 1909-2005. 
For more information, here's a link to a piece that aired recently on North Country Public Radio (http://www.northcountrypublicradio.org/news/story/17764/20110602/keene-valley-slide-accelerating-driven-by-rain). 

1 comment:

  1. I remember going to a slide with you for Forest Soils back in '91. Is this in the same area? Fascinating stuff.

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