New borehole data and trench exposures in northern Anglesey show the Gwna Group melange resting directly upon disrupted sedimentary rocks of the New Harbour Group. Field evidence from Anglesey and the Llŷn peninsula shows that units previously given formal lithostratigraphic status in the higer part of the Monian Sopergroup occur as clasts within the Gwna Group melange. me wbdivision of the Monian Supergroup is therefore simplified to comprise only three groups: the South Stack (lowest), New Har- bour, and Gwna Groups (highest). The disruption of the upper part of the New Harbour Group is interpreted to have been efferied during emplacement of the Gwna Group melange.

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