The Berw and Aber-Dinlle Faults represent the brittle manifestation of a major lineament, the Menai Strait Fault System, which is known to have been active intermittently since late Precambrian times. Documentation of historical seismicity in this area provides information on the most recent activity along this structure, although there is a paucity of field evidence to support conclusively Cenozoic or later movements. This work documents data from three separate aeromagnetic datasets, combined with field relationships, which reveal a well-defined Cenozoic dyke swarm traversing Anglesey and Snowdonia, showing clear sinistral offset (up to 1.5 km) along the Berw Fault, the most northwesterly component of the Menai Strait Fault System. These data provide conclusive evidence of Cenozoic (<65 Ma) strike-slip faulting in North Wales.

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