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Claystone deposited in Glacial Lake Missoula, Montana, USA

Claystone /ˈklstn/ is a clastic sedimentary rock composed primarily of clay-sized particles (less than 1/256 millimeter in diameter).

Claystone does not include clay shales, which are rocks that are laminated or easily split into thin layers. Claystones are also distinct from mudstones, which are partly hardened muds that slake when wetted; claystone is fully hardened material.