The Culm Measures are a thick sequence of geological strata originating during the Carboniferous Period that occur in south-west England, principally in Devon and Cornwall, now known as the Culm Supergroup. Its estimated thickness varies between 3600 m and 4750 m though intense folding complicates it at outcrop. 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).