"stable"@en . "Stratified bedrock. Occurs onshore. Deposited during the Ludlow Epoch (Silurian Period) (426.7-422.7 Ma BP)."@en . "Irfon Formation"@en . "426.7"^^ . . . "422.7"^^ . . . . . . "IRF"@en . "Irfon Formation"@en .