Some local knappable rocks

July 15, 2021

I like going out to look for knappable rocks. Mostly I like having a goal in mind when I head out for a walk—somehow “just going for a walk” seems monotonous and unappealing to me most of the time. So I pick something not-too-important and a little rare, or something that invites observation—a particular type of stone on a rocky shore, or cheap, useful textiles from a nearby thrift store, or some kind of human or natural feature to take pictures of—and often I’m out for hours, just wandering and looking at things.

Downtown Salem, where I live, is at the neck of a small peninsula. Over the last couple of weeks I’ve made several trips to most of the public parts of the coastline, trying to find types of stone that I could use for flintknapping. Flint (or chert, which is a compositionally-similar material that occurs more in North America) is not present in the local geology of Salem. An item in Science from 1963 suggests that English flint, brought as ballast on trading ships, is present in many historical northeastern harbors, and specifically mentions the Salem shoreline near the Beverly bridge, which is where the pictures below were taken.

I do not think I found any flint[1]. However, the site does have some cobbles of a fine-grained dark grey material, speckled with white, that takes a pretty nice conchoidal (shell-shaped) fracture, making it usable for knapping. These cobbles often have a whitish cortex or a very pale white-tan patina, which originally led me to think they might be flint. I believe that they are a locally-occurring rhyolite. While the 1963 article seemed conclusive that some English flints are present on the coast, this site is mentioned only in passing, and I have to wonder whether what was found was actually this material.

an axe-head shaped rock, dark grey with white flecks, held in a hand. The rock has conchoidal fractures at its edge where I knocked off flakes.

The other side of the axe-head shaped rock, held in a hand. A thin vein of a lighter-colored material runs from bottom-left to upper-right. This side has shallower flakes taken out.

A close-up of a single conchoidal (shell-shaped) flake taken off the stone pictured above. It has an extremely thin semicircular edge opposite the striking platform, which is thicker. The lighter-colored flecks in the dark-grey material don't obviously interfere with the fracture plane, which is relatively smoothly curved.

The same flake as above, but at an oblique angle, showing the fineness of the edge. While sharp, this edge is extremely delicate; a tool would be worked further to strengthen it.

An assemblage of cobbles of the dark grey material used above. Many of them have a whitish-tan patina; one seems to have a light tan cortex layer of a different material. The area these came from has been said to contain English flints brought over as ballast by trading ships; could this material have been misidentified as flint?

A moderately-large cobble of the dark-grey material, broadly cube-shaped. A brick in the background suggests that it's around 12" x 10" X 8". It has a white cortex on the side facing the viewer.

A much paler material with variegated specks an areas of color; rusty red and dark purple grey. The cobble is oblong, maybe 7" in the long dimension and 3" in the short.

Another small cobble of the dark grey material as it appears in situ. Most of what can be seen is the light tan patina; it might be mistaken for a large chunk of dusty granite gravel.

  1. On separate occasions, I’ve tried striking steel to a prepared edge of each of the rock types pictured, but I haven’t been able to get a clear spark from any of them. On the other hand, I’ve never used flint and steel, so there may simply be a problem with my technique. ↩︎