Claystone is a geological term used to describe a sedimentary rock that is composed primarily of clay-sized particles (less than 1/256 millimetre in diameter). It does not refer to those rocks that are laminated or easily split into thin layers (clay shales). Claystones are distinct from mudstones, which are partly hardened muds that slake when wetted; claystone is fully-hardened material.

sedimentary rocks

Tonstein