Shale is a fine-grained form of rock.
Some hills on Capella IV were composed of shale. In 2267 James T. Kirk and Spock used their communicators to create a sonic disruption which toppled a rock face comprised of shale. The rocks collapsed, blocking the entrance to a canyon and making it more difficult for Kras and a group of Capellans to reach Kirk and his party. (TOS: "Friday's Child")