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1747, possibly a specialized use of Middle English schale "shell, husk, pod" (late 14c.), also "fish scale," from Old English scealu (see shell (n.)) in its base sense of "thing that divides or separate," in reference to the way the rock breaks apart in layers. Cf. Middle English sheel "to shell, to take off the outer husk" (late 15c.). Geological use also possibly influenced by German Schalstein "laminated limestone," and Schalgebirge "layer of stone in stratified rock."
A sedimentary rock formed from layers of clay.