Volcanic stratigraphical architecture of the East Mendip Silurian Inlier, Somerset, UK

The results of extensive quarrying and a drilling programme since 1982 have enabled a revision of the previous work on the Inlier. Thickness variations of the volcanic units are seen to be greater than previously thought. Evidence is presented to show that the main volcanic centre lies to the east or northeast of the Inlier. The so-called ‘Vent Agglomerate’ is interpreted as subaerial volcanic debris flows infilling two steep-sided palaeo-valleys trending southwestwards and cutting into the volcanic sequence. The explanation of the Devonian (Upper Old Red Sandstone, Portishead Formation)-Silurian contact on the south side of the Inlier as a major overthrust is rejected in favour of its representing an angular unconformity involving the further folding of a pre-Upper Devonian periclinal fold by Variscan earth movements.