Bideford Black Seam, Abbotsham, North devon (P Ward 2010)
BIDEFORD BLACK is a unique, naturally occurring carbon based mineral, or culm deposit, running alongside seams of high quality anthracite (coal) across North Devon. The deposits were formed over 350 million years ago during the Carboniferous geological era on the Gwondana landmass somewhere near the equator, experiencing a sub-tropical climate. The region would have been covered in forests and lagoons inhabited by Tree ferns and other early plant and animal life. According to research conducted by local geologist Chris Cornford, the mineral is unique in that it only contains the lignum of tree ferns rather than the spores, bark and leaf matter normally associated with coal deposits. He suggests that this may have been due to the trees being naturally felled and trapped in ox-bow lakes (indicated by the lens-shaped pockets where it is now found) where the outer layers were removed before being deposited and trapped beneath layers of landslip soil and rock. The deposits were then crushed and taken down to 8km beneath the earth’s surface as plate tectonics took its part, grinding and compressing the fine vegetable matter to form the greasy clay deposits we find today. The mineral is made up of flat hexagonal platelets, similar to that of graphite that is used in pencils. This structure may have been exaggerated by the shearing and sluicing action of the earth as it moved deep within the earth. The mineral consists of roughly equal parts of carbon, silica and alumina – the carbon providing the exceptionally rich black colouration that made it so popular as a pigment. The seams stretch from Hartland and Abbotsham on the coast in a southeasterly direction beneath Bideford and inland as far as Umberleigh.
Tree fern photographed at Tresco Abbey Gardens, Isles of Scilly (P Ward 2011)
Geological map showing BIDEFORD BLACK deposits (from BIDEFORD BLACK - THE HISTORY of a UNIQUE LOCAL INDUSTRY, published by SOUND ARCHIVES NORTH DEVON, 1994.)
Lens (Bideford Black on paper, a painting by P Ward, 2008)
While Chris has conducted extensive and comprehensive research into BIDEFORD BLACK, geology is by no means a finite science and any other ideas or thoughts about its physical and chemical nature and formation are more than welcome...