 They are so called because of the occasional presence in the Barnstaple–Hartland area of a soft, often lenticular, sooty coal, which is known in Devon as "culm".The word culm may be derived from the Old English word for coal "col" or from the Welsh word "cwlwm" meaning knot (due to the folding of the beds in which the coal is found).Most of the succession consists of shales and thin sandstones, but there are also occurrences of slate, limestone and chert.