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At the time of the report on Cambrian correlation by Cowie et al. (1972), the pre-Ordovician rocks of Anglesey, the ‘Mona Complex’, were generally accepted as being of Precambrian age. Radical re-interpretation of the Mona Complex followed from palaeontological evidence reported by Muir et al. (1979) and from a plate-tectonic evaluation by Barber & Max (1979), and since then strong evidence supporting the Cambrian age of a substantial part of the Mona Complex, the Monian Supergroup, has been published. Summary accounts of the Mona complexes (or terranes) are to be found in works dealing primarily with Precambrian or Neoproterozoic rocks: Gibbons et al. in Gibbons & Harris (1994), Horák & Gibbons in Carney et al. (2000), and McIlroy & Horák in Brenchley & Rawson (2006). In these reviews the former Mona Complex is treated as three fault-bound complexes, of which the Coedana Complex, of granite and gneiss, and the Eastern Schist Belt (Aethwy Terrane), are accepted as of Neoproterozoic age, whereas the Monian Supergroup, a thick metasedimentary succession, is considered to be of Cambrian age (Fig. 13).

The lowest division of the Monian Supergroup (column 20), the South Stack Group, has yielded detrital zircons that gave an age of 522 + 6 Ma, that is, well within the Cambrian, and the strata cannot be older than that.

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