Fr.: schiste, schiste argileux
A fissile rock composed mostly of layers of clay-like, fine-grained → sediments. Shale is the most frequently occurring → sedimentary rock.
Probably from obsolete or dialect shale "scale, shell," from M.E., from O.E. scealu, → rock.
Ardavâl "shale," in the dialectal Mod.Pers. of Golpâyegân, Arâk, Xonsâr, etc.