A 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 the Dicranograptus clingani and Pleurograptus linearis zones overlain by Ashgill mudstones with Phillipsinella parabola.
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Janice Harper (1954) studied this question.
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