Day #103 Deskcrop: Biotite Phyllite, Walloomsac Fm.

We’ve arrived in the biotite zone (just barely). Today’s deskcrop comes from the Middle Ordovician Wolloomsac Formation in Dutchess County, New York. This deskcrop was metamorphosed during the Taconic and Acadian Orogenies and is just inside the mapped biotite isograd. There’s supposed to be macroscopically visible porphyroblasts of biotite, but I’m hard pressed to recognizing them, myself.

Tomorrow we’ll move on to the garnet zone…

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Biotite Phyllite, Walloomsac Fm.

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