Culm Supergroup

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 3600m and 4750m 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).

The majority of the succession consists of shales and thin sandstones, but there are also occurrences of slate, limestone and chert.

Culm grassland on the formation's slates and shales is composed of purple moor grass and rush pasture. It is noted for a wide diversity of species, some extremely rare including the marsh fritillary butterfly. Some 92 percent of Culm grassland has been lost in the past 100 years, 48 percent being lost between 1984 and 1991 alone. There are a number of organisations trying to halt the decline including Devon Wildlife Trust with its Culm Natural Networks project, Butterfly Conservation, and Natural England with its Environmental Stewardship Scheme.

Culm soils have traditionally been used for grazing as they are heavy to work and acidic.

In the main Culm Basin and north Devon the succession is nowadays divided into an upper Holsworthy Group and an underlying Teign Valley Group (formerly and popularly, the 'Lower Culm'). By way of contrast, in south Devon the entire supergroup is represented by the Chudleigh Group. The Holsworthy Group is itself divided into an upper Bude Formation and a lower Crackington Formation though a Bideford Formation intervenes in the Bideford area. In the Launceston area the entire group is represented by the Bealsmill Formation. The Teign Valley Group is subdivided into numerous formations, two of the more significant of which are the Dowhills Mudstone and Teign Chert formations. The Chudleigh Group divides into an upper Ugbrooke Sandstone Formation and a lower Winstow Chert Formation.

...
Wikipedia