Day #114 Outcrop: Alexandria Bay Nonconformity

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.

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Explore this and all of my 2010 Deskcrops and Outcrops in Google Earth!

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