In the type area, and southwest of it, the Capel Curig Volcanic Formation comprises up to three ash-flow tuffs interbedded with marine sediments. The tuffs are generally massive and welded, but pass laterally and upwards into current bedded, ripple marked tuffs. They are believed to have been deposited in a subaqueous environment, after originating from subaerial eruptions farther north where the Formation is represented by an uninterrupted sequence of intensely welded ash-flows. The lower contacts of the submarine ash-flows contrast with subaerial ash-flows in being welded and by locally transgressing the underlying sediments at angles up to 90°, with minor apophyses resembling magmatic intrusions. Eutaxitic foliation, however, remains generally parallel to the regional dip. The sediments adjacent to the transgressive undersurfaces are disturbed and reconstituted. The irregular contacts, together with adjacent discrete pipe-like tuff bodies are interpreted as down-sags analogous to load casts but of unprecedentedly large scale, up to 100 m, formed by the liquefaction and yielding of the sediments during and after the emplacement of the tuffs.

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