claystone, hardened clay. Some geologists further restrict the term to a sedimentary rock that is composed primarily of clay-sized particles (less than 1/256 millimetre in diameter) and is not laminated or easily split into thin layers; such rocks that show cleavage roughly parallel to the bedding plane often are classed as clay shales. Claystones that are massive and blocky are sometimes called mudstones, but some geologists class as mudstones partly hardened muds that slake when wetted, reserving the term claystone for fully hardened material.