Wednesday, November 20, 2013

Ausable Chasm

Earlier in the fall (during peak leaf season), my family and I took a two-day drive up to the Plattsburgh area in New York, then across the top of Lake Champlain (spitting distance from Canada) and down through Vermont.  We obviously saw a lot of pretty trees changing colors.

Since I'm a geologist, I also subjected my long-suffering family to two geological areas (one more touristy than the other) - Ausable Chasm and the Chazy Reef.  Today I'll talk a bit about Ausable Chasm (and will tackle the Chazy Reef on another day).

Ausable Chasm is a sandstone gorge through which the Ausable River (Ausable is French for "of sand") runs for a couple of miles (~ 3 km) from the area just northeast of Keeseville, NY toward Lake Champlain.  The gorge is as deep as 300-500 feet (90-150 m) in places and only 20-50 feet (6-15 m) wide.  It's a popular summer tourist attraction with walkways and overlooks through the gorge.

Not a good place for those who don't like stairs

Geologically, the gorge is pretty simple.  The Ausable River carved a gorge a little over a mile long (~2 km) down through the Cambrian-Period (about 500 million-years-old) Potsdam Sandstone since the end of the Pleistocene Epoch ice age (which ended up there around 10,000 years ago).  It was basically due to the headward erosion of ancestral Rainbow Falls which is today located near the visitor's center.

Peaceful waterfalls relentlessly cutting

A little math, just for fun...  2 km / 10,000 yr = 0.0002 km/yr = 20 cm/yr (about 8 in/yr).  To tell the truth, that seems a bit high to me but we have to realize that as the ice age was ending, there were tremendous volumes of meltwater flowing off the massive continental glaciers and water volume (and erosion) would have been much greater in the past.  Remember, while we calculate an average, erosion is discontinuous in time - for example, 100-year floods cause massive amounts of erosion in a day or two (read about the Devastating Floods of 1996 at Ausable Chasm).  Hiking the gorge this year, I still saw plenty of evidence of those floods.

I-beam from destroyed bridge in side canyon

Ausable Chasm was first seen by non-Native Americans in 1765 and, since then, has been a draw to tourists in the eastern Adirondacks.  Landscape photographer Seneca Ray Stoddard took a number of stereographic photos of the chasm in the post-Civil War era which also publicized the site (note that walkways existed through the gorge even back then).

The path of the gorge seems to be controlled in places by joints (vertical fractures in the sandstone) and faults.  A number of side canyons have also eroded out along those fractures (as in the I-beam picture I showed above).  This is especially true near the end of the chasm where the river takes a couple of extreme right-angle turns and is very narrow.

From Google Maps
The straight and narrow part at the end

In a few places, the river even changed course in the past and left behind dry canyons you can hike through (and we did).

Now a few words about the Potsdam Sandstone which makes up the walls of the chasm.  It's a great story and I could go on for pages but I'll just give a quick summary here.  Over a billion years ago, a microcontinent collided with proto-North America and thrust up a Himalayan-scale mountain range (exactly the same as India's recent collision with Asia forming the actual Himalaya).  These mountains eventually eroded away over hundreds of millions of years (as will the Himalaya - nothing lasts forever).  This eroded surface became the basement rock of New York State.

About 550 million years ago, sea levels rose and ocean water started lapping onto that bare metamorphic bedrock (no life existed on land yet).  First a sandy beach, then a shallow seas.  These sands eventually lithified into sandstone.  We call this the Potsdam Sandstone in upstate New York.

The lower part of the Potsdam contains pebbles and fragments of the underlying metamorphic basement rock.  Lower parts of the sandstone also show cross bedding indicating deposition in an environment of braided rivers and streams.  It then becomes more of a pure sandstone (in the past it was mined for its pure quartz composition) as you move upwards which has some ripple marks and a few marine fossils indicating a shallow sea.

See the ripples on the sandstone bedding plane?

Trace fossils are more common than actual body fossils (although trilobites and even a rare jellyfish fossil has been found in the Potsdam).  Here are some trackways from an ancient arthropod (there's no consensus among paleontologists about what made this track, even if it was an arthropod or a type of molluscan) walking on the seafloor (it's a slab on display near the visitor center).
Here's one of the coolest fossils from the Potsdam.  It's now on display in the New York State Museum in Albany.  These trackways on ripple-marked sandstone (looking like motorcycle tracks) were made by a large arthropod scooting along the bottom of the sea half a billion years ago.
Climactichnites tracks in the New York State Museum
Yea, yea, I know what you're thinking right about now if you're interested (and interesting - I'd probably get along well with readers who read my rambling posts on this blog) enough to have read this far.  You're thinking how I wrote this whole post of Ausable Chasm and haven't put in one "traditional" photo of the main part of the chasm.  Ok, here you go.
Pretty place
Being the curmudgeon that I am, let me add that all the websites and guidebooks rave about the elephant's head rock formation at Ausable.  Somehow we missed it.  I don't care.  I'm far more interested in looking at the real features in the rocks, the features than mean something (like ripple marks, for example), rather than artifacts of some promoter's often-limited imagination.  It always irritates me when guidebooks to natural places like gorges and caves make up cute names for all the rock features ("goblin's nose" or "devil's den") yet don't explain at least a little of the science of what they're seeing.  These aren't just featureless gray rocks that happen to resemble something.  The Potsdam Sandstone represents the first influx of an ancient sea on a rocky, barren landscape devoid of terrestrial life half a billion years ago.  That's far more interesting to me that a rock that looks like a fucking frog.  I know I'm a dying breed though when I see that Ausable Chasm has been adding all kinds of activities (tubing, rapelling, zip lines across the canyon, for example) to appeal to dim-witted, adventure seeking tourists who could care less what the hell they're zipping across.  The slow Disneyfication of America (nothing against Disney, but does every place now have to feature thrill rides to be interesting to people?).
Some grumpy old bastard with no sense of fashion looking at rocks
"As long as I live, I'll hear waterfalls and birds and winds sing. I'll interpret the rocks, learn the language of flood, storm, and the avalanche. I'll acquaint myself with the glaciers and wild gardens, and get as near the heart of the world as I can." - John Muir (1871)


  1. Steve, I have similar curmudgeonous thoughts. Add these: "natural" history guidebooks that have the barest and minimal insight into any of the actual geological underpinnings for the stuff they're waxing eloquently about. Regardless, of that, thanks much for the tour of AC, something that I have somehow missed despite having grown up 85 miles south of there and having studied geology at the college that bears the name of rock so well displayed there.

  2. I always enjoy your explanations! How about including Mrs. HVG in your photos now and then -- or some of the younger hvgs?

  3. Great post! I lived in Potsdam NY for many years after getting my Geology degree at the state university there. We were very proud of our native rock, even going so far as to call the local HS sports team the Potsdam Sandstoners ('Stoners for sure, and you can imagine the kids had a field day with that name). It's a beautiful formation, and if you drive west from the town you can go backwards in time lithologically. I'll have to bring the family to the chasm sometime so I can wax poetic about the sandstone there. Thanks for a great post!

  4. Hi Blinky. When were you there? I finished '77. Probably went on all the same field trips to "The Snake," Alex Bay, Poplar Hill, etc. with Brad, Jim, Neal and Bill (with Frank back with the seismographs in Timerman).

  5. I will be going there in the next few days. I will definitely be looking for cross bedding, not elephant heads! :)

    1. January 16, 2016- My husband and I just hiked through Ausable Chasm today, wearing the rented slip on micro spikes to avoid slipping over the edge on the icy trails. Amazing! We are awed by the geology. We were wondering if anyone could tell us how Jacob's Well was formed.