. . "Brithion Tuff [Obsolete: use BRT]"@en . "Volcanic breccias and felsic, welded ash-flow tuffs."@en . "Sharp but conformable contact with underlying mudstones of the Aran Volcanic Group."@en . "Creigiau Brithion Formation [Obsolete: use BRT]"@en . . . . . "Tuff with subsidiary breccia. Stratified bedrock. Occurs onshore. Deposited during the Llanvirn Series (Ordovician Period) (468-458.2 Ma BP)."@en . "Brithion Formation"@en . "To 75m, but the formation wedges out southwestwards and northwards."@en . "Brithion Formation"@en . . . "BRT"@en . . "stable"@en . "458.2"^^ . "Conformably overlain by undivided siltstones of the Aran Volcanic Group."@en . . "468.0"^^ .