The Townsend Tuff Bed has a highly distinctive internal stratigraphy which allows it to be recognized at a large number of localities scattered over the Welsh Borders and South Wales. It is a group of 3 almost directly superimposed, normally graded air-fall tuffs that originated in distant but powerful Plinian eruptions (centre unknown) and became spread over extensive, but practically featureless, coastal mud flats. The bed occurs in a barren interval within the early Lower Old Red Sandstone, between a Silurian invertebrate-vertebrate fauna below and an already Devonian vertebrate assemblage above. The base of the Devonian System in South Wales and the Welsh Borders is accordingly defined as occurring at a regionally uniform lithological contact within the Townsend Tuff Bed.

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