The stratigraphy and structure of the Devonian Rocks in the Brendon Hills, West Somerset

Subdivisions of the Middle-Upper Devonian strata—Hangman Grits, Ilfracombe Beds and Morte Slates—have been mapped. The Ilfracombe Beds (Givetian-?Lower Frasnian) are composed of slates, cleaved siltstones, sandstones and limestones. Lower and middle parts of the Morte Slates (?Frasnian) consist of sparsely fossiliferous, non-calcareous slates, cleaved siltstones and subordinate sandstones. Estimated maximum thicknesses are 2800 feet for the Ilfracombe Beds, and 2000 feet for the lower and middle parts of Morte Slates. Upper Hangman Grits accumulated in non-marine, possibly Old Red Sandstone depositional conditions; Ilfracombe Beds and Morte Slates were deposited, possibly rapidly, in shallow seas. The strata have been subjected to cleavage folding, flexure folding and faulting. Cleavage folds (major and minor) are tight, asymmetrical to overturned, having associated axial-plane cleavage, bedding/cleavage lineation and jointing. The flexure folds, in contrast, are broad, open and symmetrical. Both cleavage and flexure folds have axes which plunge gently east to east-south-eastwards. Two periods of folding appear to be represented: first, cleavage folding and, secondly, flexure folding accompanied by the rotation of cleavage. It is tentatively suggested that flexure folds were produced as broad buckles in the overriding mass of a major thrust. The major faults strike in a north-west to northerly direction. The Timberscombe Fault System has both dextral horizontal and vertical displacements, which developed mainly prior to New Red Sandstone times.