An indurated clay having a texture and composition of shale but lacking its fine lamination or fissility

a sedimentary rock composed of extremely fine grains.

a sedimentary rock composed predominantly of particles less than silt size usually comprising clay minerals.

Claystone (IPA: ) 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.