AbstractA section in the River Dwyfach shows the main Snowdon Lavas and Tuffs to be overlain by a series of sediments and tuffs with a Longvillian fauna near the base. These sediments are followed by black graptolitic shales belonging to theDicranograptus clinganiandPleurograptus lineariszones overlain by Ashgill mudstones withPhillipsinella parabola.