The East Irish basin is characterized by three sets of faults, striking N-S, NE-SW and E-W. The Menai Strait fault system bounds the basin to the south and shows pre-Caledonian to Cenozoic movement. This span of activity contrasts with similarly oriented faults, to the north of the basin, which did not exist before the early Carboniferous. N-S faults have a similarly diverse history. Some may have influenced Silurian sedimentation and controlled the disposition of Ordovician volcanic centres in Snowdonia, a contrast with N-S-striking faults in the north of the area which originated as steep strike-slip faults during the late-Silurian. E-W faults control Dinantian and Permo-Triassic thickness variations although, unlike the other fault trends, pre-Carboniferous precursors are not apparent. Some of the basin-bounding faults developed as wholly new faults during Permo-Triassic extension.