AbstractRefined biostratigraphy for the Arenig Series has permitted the use of the three stages recognized in South Wales, and hence the recognition of structurally‐bounded areas of North Wales with substantially different depositional histories. During the Moridunian and Whitlandian, continuous subsidence was restricted to the western margin of the present mainland, and in many areas deposition did not commence until the Fennian. Volcanics, mass flow deposits, and synsedimentary deformation indicate two minor phases of tectonic activity. Late in the Moridunian, uplift around the Harlech Dome caused the absence of the Whitlandian. During the Fennian rapid fault‐controlled subsidence occurred in the west allowing the incursion of an open ocean biofac