1 April 2013

“Outcrops” on the barn at Peirce Mill, DC

Posted by Callan Bentley

The Friday before last, I was in DC for a fun geology/botany field trip, and I got the opportunity to stroll around the barn at historic Peirce Mill, a historical grain mill along Rock Creek in Rock Creek Park, Washington, DC. The barn is immediately south of Tilden Street NW.

It appears to have been constructed from local stone: metamorphic rocks of the Rock Creek Shear Zone, a ductile fault that trends north-south through DC’s exposures of Piedmont rocks. Strain is variable in these rocks, but can be quite high. Here’s a flattened clast with an axial ratio (on this surface) of 16:1.

…Some tailed porphyroclasts are discernible in that shot, too.

Here’s another nice block:

Zooming in closer….

If your eye is having a hard time making sense of that, it’s probably because the surface has its own morphology, distinct from the fabric of the rock within. The surface bears slickenlines, grooves gouged into the rock during faulting. “Beneath” that is a more or less horizontal foliation, defined by the shape of the grains. I’ve outlined a few ribbony looking quartz grains in there.

Last, consider this block, which displays gumball-sized garnet porphyroblasts, as well as oat-flake-sized porphyroblasts of chlorite:

What cool building stones have you seen lately?