The study of the foraminiferal fauna in the Ogmore section in south Wales produced a detailed late Chadian–early Arundian biostratigraphy and sequence stratigraphy with a higher resolution than in the Arundian stratotype in Hobbyhorse Bay. The earliest Visean (late Chadian) pedogenic horizon close to the Gully Oolite and Caswell Bay Mudstone boundary (MFZ9) corresponds to the mid-Avonian unconformity. The entry of primitive archaediscids of MFZ10 within the Caswell Bay Mudstone is followed by more advanced archaediscids of MFZ11 at the base of the High Tor Limestone. The mid-Avonian unconformity at the Chadian–Arundian boundary is not confined to the London–Brabant Massif, but is also present in central Europe, the East European Platform, the Urals, west Siberia and probably also in Kuzbass and North America. The global extent of the the mid-Avonian unconformity suggests that it is a consequence of early to mid-Visean glacioeustacy.