Shale, the Shy Rock

I was poking around the hills in my city the other day, investigating a sandstone. A few steps beyond the edge of the sandstone, the roadside showed no rocks whatever, just loose soil. But eventually I found a hard pod—a concretion—and pounded it open to find pure clay powder inside. That's how I knew the hillside was shale. Shale is what clay becomes when you bury it deep enough, and shale is usually eager to turn back into clay. This picture of shale is from a roadcut, and after only a few years in the sun and rain it would crumble at a touch. Shale wants to hide. That's why roadcuts are so important for getting fresh samples, even of reluctant rocks like shale.

Shale — Geology Guide photo