30 June 2023

Friday fold: Wissahickon block

Posted by Callan Bentley

Click to enlarge

I’m writing this on the Amtrak train from New York back to Charlottesville, traveling (again) under orange-hazy skies due to Canadian wildfire smoke. I took my son to Manhattan for a concert, and we stopped off en route for an overnight in Philadelphia, visiting a former student of mine who’s now an assistant professor of geology at Rowan University in New Jersey. While walking around Philly, I saw an old church that had been built from blocks of locally-quarried Wissahickon Formation schist. This particular block shows some nice small-scale folds in both the foliation (subtle, at right) and a trio of quartz veins (more obvious, center).

Also, in another block of the same formation in the same church wall, I saw this lovely little garnet porphyroblast bearing pressure-shadow “beards” of quartz that have grown out along the plane of foliation: