Monday, October 13, 2014

Another Roadside Attraction

     We all have a story. You, me, rocks.  Go ahead and tell your story, it's important.  Mine still needs a little editing.  That leaves rocks.  Let's talk about the life of rocks.  They're born, things happen to them and then (how to put this?) they're recycled.  It's a fascinating tale of sedimentation, lithification, volcanism, tectonics, orogeny, metamorphism, subduction, erosion and quarrying.
     Washington County (W. C.) has an intriguing mix of rocks.  There is the very old, metamorphosized Adirondack basement suite:  gneisses, marbles, meta-gabbros and quartzites found in the northwest part of the county.  In the central Hudson/Champlain lowlands we find sedimentary deposits of limestone, dolostone, sandstone and shale.  Rising above the valley and extending into
Vermont are the Taconic hills with their peripatetic slates and mudstones that moved into the neighborhood from further east.
     While the county is completely underlain by bedrock, it's not always easy to see.  Much of it is covered by soil, water and vegetation.  Think of a beautiful woman:  her clothes may be fabulous but they also hide interesting things. Here in WC we have more exposed rock than Iowa but less than Arizona.
     Geology and the American way of life intersect at the road cut.  It's where construction crews have blasted and bulldozed whatever got in the way of their straight and level creations.  Not only does this make travel faster and safer, it gives us little peeks into the earth's crust, all from the comfort of our cars. With apologies to Tom Robbins for my title, I want to look at some of these easily accessible outcrops in this and future posts.
     The twisted sister of road cuts is located on County Road 61 about 1 1/2 miles east of Shushan in the Town of Salem.  One look at this small, south-facing exposure and you know the rocks have a story to tell; dramatic, even violent.  What you see is many thin layers of black fine-grained mudstone bent, folded and broken.  After consulting various geological sources,  I came up with the following biography.
     This outcrop is located in the Taconic Mountains, so we know that it's more than 450 million years old.  The rocks originated when sediments from an early incarnation of North America were eroded and washed into the sea where they finally settled to the bottom on the slope/rise edge of the continent. This would place them in what is now eastern Vermont.  As this mud piled up, its weight pressed out water and cemented the clay particles together to form solid rock (lithification).  Following our story analogy, you could say the rock was "born."  Life was ho-hum for many millions of years with continued deposition burying the layers deeper beneath the sea.
     Eventually there came a pressure from the east.  This compression resulted as two of earth's tectonic plates came together with one subducting under the other.  The plate drifting in from the east carried a string of large islands (think something like Japan or Indonesia) and there was a very slow motion collision lasting 10 to 20 million years that built up a huge mountain range along the edge of the continent.  This event is called the Taconic orogeny and our little outcrop got caught in the middle of it.
     As the island arc slid up onto the edge of proto-North America it bulldozed a huge pile of crust ahead of it.  Deep in this accretionary prism, heat and pressure were high enough to soften and deform what had been hard, horizontal layers.  Strata folded, faulted and slid past each other.  It was a huge mess.
     Gradually the collision used up its energy and mountain building came to an end.  Then erosion took over.  Lots and lots of erosion over hundreds of millions of years stripping away vertical miles of the Taconics until we are left with today's scenic but modest range.  Driving along Rt. 313 from Cambridge, NY to Arlington, VT takes you into the carved out core of the Taconics, courtesy of the Battenkill River's relentless downcutting. The twisted sister outcrop shows us what it's like to be pushed fifty miles west with the weight of the world on your shoulders.  Stop sometime and listen to her story.

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