Day #87 Outcrop: Glacially Polished Potsdam Sandstone

I’ve recently been uploading field photos I took early last summer and one of those outcrops is just crying out to be blogged about. This outcrop is located in the St. Lawrence River Valley in upstate New York, northeast of Alexandria Bay in the Thousand Islands region. The bedrock here is Cambrian Potsdam Sandstone and the outcrop in question is a great lesson in glacial processes. The top of the outcrop is beveled off and polished by glacial scouring. The surface was actually quite slippery with a thin film of water from a passing rain shower.

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Glacially Polished Potsdam Sandstone

Looking at the surface in detail, you can see both glacial striations (right) and chatter marks (left) side-by-side in the image below. Not much question which way the ice sheet was moving here…

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Glacial Chatter Marks (left) and Striations (right)

Finally, there are some remarkable joint patterns exposed on the polished surface of this outcrop, just so the structural geologists don’t feel left out.

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Remarkable Jointing Pattern

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

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