Wissahickon Park, Philidelphia, PA

Wissahickon Park is located on the outskirts of Philidelphia, PA. In this site evidence of ductile sinistril (left lateral) shearing is present along the sides of a river that flows through and created the valley which situates the park. Within the shear zone many complex structures developed and can be seen by taking a short walk through certain paths. The age of rock and metamorphism is concurrent with the Taconic Orogeny of the Ordovician Period in the Paleozoic Era.

Taconic Orogeny

The Taconic Orogeny was the collision of a volcanic island arc. Also, results are seen in Central Park, Manhattan, New York. Deformation from this is still observed up and down the East Coast.
The bedrock of Wissahickon is many grades of Schist. An abundance of Garnets are scattered throughout the area ranging in all sizes. All of this was miles beneath a fault zone. The depth at which these rocks deformed was subject to intense heat and pressure and did not allow for brittle, sudden offsets, like you would find closer to the surface. Instead, the area behaved like plastic, sort of like a taffy.

Garnets Thumbnail.

Garnets are commonly found throughout the entire park at many different sizes. some can be as big as golf balls! These and other porphyroblasts (crystals in a metamorphic rock) such as staurolite grew during heavy compression of the schist.

Structural Feature Thumbnails

All of these are located within the walls of the gorge cut by the river through the park, and many more remain. Click to enlarge and view.

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1. Brittle/Plastic

Note the discontinuous white band. This is a quartz vein which has been offset by the shearing of the rock. The Quartz was too rigid and had a more resistent behavior than the surrounding schist, and behaved as if it were a pencil stuck within a mass of silly putty, and having the stuff smushed around. It is broken off from it's original trace.

2. Boudin Structure

Next to the cigarette lighter, see how a quartz vein is stretched apart within the schist. Again, the more resistent quartz within the plastic schist.

3. Migmatite

My sister-equivalent, Adrienne (see acknowledgements) points out a migmatite. This is the highest level of metamorphism in the park. The rock here has been so deformed and heated that it partially melted and became granite.

4. Folding

Witness here a small fold next to the cigarette (don't smoke its bad for you!). You get a sense for the shear direction and can see how highly metamorphosed the rock surrounding it is. The darker band is a highly sheared zone as well.

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