Thursday, October 13, 2016

Wissa-hikin' on the Wissahickon: awwww, schist!

This last weekend I went on a hike up at Wissahickon Park to check out the local geology with the Temple University Geology Club.

Location of Wissahickon Park
Location of Wissahickon Park.

I've been through the Wissahickon plenty of times, as it's one of my favorite places to go biking, but this was the first time I'd ever walked around. I didn't take many pictures, but it's a beautiful park, and a nice little nature preserve for being this close to central Philadelphia.

Wissahickon Park

Wissahickon Park

Wissahickon Park

Wissahickon Park

The exposed bedrock at the park is the aptly-named Wissahickon Formation, a glittery schist which has been widely used as a decorative building stone for Philadelphia from colonial times to today.

Neocolonial home around Philadelphia.

Temple University Baptist Temple
Temple University's Baptist Temple, which the university is named for, constructed with Wissahickon Schist building stone.

Schist is a metamorphic rock, meaning it has been altered to it's current state through immense heat and pressure. Before metamorphism, the rock it used to be was likely shale, a sedimentary rock formed from the lithification of clay and mud. This former rock was formed during the Cambrian period, around 500 million years ago, when this part of the continent was not only far out to see, but south of the equator and rotated about 90 degrees from today (lots can change in 500 million years).

North America during the Cambrian
Location (red star) of sediments deposited which would eventually metamorphose into the Wissahickon Schist.

See the island chain just to the south (right) of Philadelphia in the above photo? That's an island arc chain, the result of two continental plates converging (colliding), producing a chain of volcanoes along the plate boundary. As this plate collision progressed as one plate was subducted under the other, eventually the continental crust that Philadelphia was apart of collided with the volcanic chain, resulting in a mountain chain, and deforming the local rocks and sediments. This was known as the Taconic Orogeny:

Taconic Orogeny

Both the stresses from the collision, along with deep burial under a mountain range, resulted in intense metamorphism of the previous rocks. For what is currently the Philadelphia area, this resulted in the formation of the Wissahickon Schist. A further 450 million years of weathering eroded down the mountain and exposed the highly metamorphosed rocks at their hearts, which is what we now see at the surface around Philadelphia.

There was a lot of interesting mineralogy and deformation textures to see at the park, but I just brought one sample home. The Wissahickon Schist is widely known for being fairly rich in garnets, which is what I was looking for.

Wissahickon Schist with garnet
Wissahickon Schist with garnets. Origin of Species for scale.

Wissahickon Schist with garnet
Wissahickon Schist with garnets. Stubby fingers for scale.