As promised back on Tuesday, this weekend I’m going to visit a pair of outcrops (well, roadcuts, actually) on opposite sides of the Adirondack Mountains that expose the Great Unconformity where the Cambrian Potsdam Sandstone lies depositionally atop a nonconformity developed on Late Proterozoic gneisses of the Adirondacks.
We’ll begin on the west side of the Adirondacks in the St. Lawrence River Valley. Today’s “outcrop” is actually a long roadcut just northeast of Alexandria Bay, NY and just west of Cranberry Creek. The unconformity surface is near the base of this cut and there is a considerable thickness of Potsdam Sandstone overlying it. The contact itself is somewhat weathered, so it’s not the best place to put your finger on a gap in time of hundreds of millions of years, though that is what it represents. If you look carefully you’ll note there’s some paleotopography on this unconformity surface.
Alexandria Bay Nonconformity
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