The ashgill rocks of the glyn ceiriog district. North wales

Abstract

AbstractThe Ashgill rocks of the Glyn Ceiriog district comprise a relatively thick sequence of mudstones and siltstones with subordinate sandstones and locally developed limestone members. Closely related deposits are found in the inliers of Cym‐y‐brain and Mynydd Cricor to the north. Correlation with other areas of Ashgill rocks suggests that the Dolhir Formation, of the Glyn Ceiriog district, can be dated as mid‐Cautlcyan to Rawtheyan while the overlying Glyn Formation is high Rawtheyan to Himantian. In the inliers to the north, that part of the Cyrn‐y‐brain Formation which is exposed is dated as Rawtheyan and both the Plas Uchaf Formation (Cym‐y‐brain) and Cricor Formation (Mynydd Cricor) are taken to be Hirnantian.The succession is interpreted as a transgressive‐regressive sequence in which shallow shelf mud‐stones and siltstones with storm‐sand and coquinid layers are overlain by shallow water shoreline deposits.Although the base of the Ashgill is not exposed in the inliers to the north, in the Glyn Ceiriog district it is seen to rest unconformably on older Ordovician strata; in all three areas the Ashgill is apparently conformably overlain by Lower Llandovery shales and sandstones.