5 June 2013
That’s a cobble of quartzite from the Potomac Formation gravels (Cretaceous aged deposits) exposed along the northern side of Piney Branch valley in northwest Washington, DC.
Some of these cobbles come bearing a load of Skolithos trace fossils:
So that’s likely to be the Cambrian-aged Antietam Formation. But we didn’t come here for the Skolithos. We came for the artifacts. These quartzite cobbles were quarried by Native Americans and used as the base material for sharp tools and implements. The quartzite breaks with a conchoidal fracture, and you can find many “worked” cobbles at this site, showing multiple intersecting conchoidal fractures. Here’s one:
This results in the production of copious flakes of quartzite:
The famous ethnologist William Henry Holmes studied this site in the late 1800s, and his work is still quite accessible and enjoyable to read.
And here’s another worked cobble, with an annotated copy following immediately to show the numerous sites were six different flakes were knocked off:
Where trees have fallen over, their roots bring up samples of both source pebbles/cobbles and anthropogenic flakes:
Another tree; same situation:
This site is on a bland-looking hillside, less than a hundred meters from the busy thoroughfare of 16th St. NW, above a nasty-looking (and smelling) stretch of urban stream. It’s a treasure trove of prehistorical archaeology, though hardly pristine. If you visit, remember not to remove any of the artifacts you find – please leave them there for other folks who want to connect with the very different world of the pre-colonial past.