U–Th–Pb Secondary Ion Mass Spectrometry (SIMS) data from detrital zircons extracted from the South Stack Group, Anglesey, UK, indicate that: (1) the maximum depositional age of the Holyhead Formation (South Stack Group, Monian Supergroup) is 501 ± 10 Ma; (2) the Monian Supergroup was deposited between c. 500 and 475 Ma and is part of the Cambrian–Lower Ordovician succession found in southern Britain and Ireland; (3) Avalonia was a major sediment source (age maxima at 543–552 and 604–627 Ma); (4) Amazonia probably also provided zircons (common Neoarchaean–Mesoproterozoic grains) weakening suggestions that Avalonia had rifted off Gondwana by Cambrian times.

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