The ashgill rocks of the glyn ceiriog district. North wales

Abstract

The Ashgill rocks of the Glyn Cciriog district comprise a relatively thick sequence of mudstones and siltstones with subordinate sandstones and locally developed limestone mernben. closely related deposits are found in the inlien of Cyrn-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 rnid-Cautleyan to Rawtheyan while the overlying Glyn Formation is high Rawtheyan to Hirnantian. In the inlien to the north, that part of the Cp-y-brain Formation which is exposed is dated as Rawtheyan and both the Plas Uchaf Formation (Cp-y-brain) and Cncor Formation (Mynydd Cricor) are taken to be Hirnantian.The succession is interpreted as a transgrcssivcrcgressive sequence in which shaIlow shelf mudstones and siltstones with storm-sand and coquinid layers are overlain by shallow water shoreline deposits.Although thc base of the Ashgill is not exposed in the inlien to the north, in the Glyn Cciriog 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.