Shale

shale, a very fine-grained sedimentary rock, usually dark-coloured, which is weak and easily split because of the way the clayey sediment has settled, so that shale always breaks roughly parallel to the stratification of the sedimentary rock, unlike some metamorphic rocks (see slate and, especially, schist). The splintered fragments are flaky and irrregular (unlike cleaved slate). Shale weathers easily, to give clay-rich soils. See also geology.