A steep fault system runs parallel to the Menai Strait, which separates Anglesey from the mainland of north Wales. The Menai Strait fault system is interpreted as a major terrane boundary along which terrane docking took place during the late Precambrian or earliest Cambrian. The evidence is based on (1) marked geologic dissimilarities exhibited by the pre-late Early Cambrian basement to these terranes and (2) the existence of early, major ductile shear zones within the fault system. Rocks as varied as blueschists, low-grade melange, and shallow-level granitic complexes were juxtaposed along these steep shear zones prior to the eruption of ash-flow tuffs and deposition of upper Lower Cambrian marine sediments in the Welsh basin. Since the amalgamation of the Anglesey area with the Avalonian terrane (then a part of Gondwana), frequent Phanerozoic reactivation of the resulting composite terrane boundary has been restricted to mostly dip-slip fault movements.