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Ordovician rocks extend from the St David’s area in the SW, along the complex Tywi lineament towards the Rhayader area in central Wales (Fig.6).On the southern side of this belt all the series of the Ordovician are developed and show a variety of facies. Correlation is generally well established. The rocks were deposited towards the edge of the Welsh trough, and the periodic influence of shallow water facies produced by positive areas to the south are manifest. However, the Carmarthen Whitland area has the most complete Ordovician succession in the British Isles (Fortey & Cocks 1986), possibly reflecting its position near the passive margin of an opening Rheic Ocean. Strata to the NW of the Tywi lineament mostly comprise deep water turbidites and graptolitic argillites of Caradoc-Ashgill age. The Gwynedd Supergroup of Woodcock (1990) embraces the Arenig to Caradoc strata of South Wales; the upper limit of the supergroup is at the level of the sub-Ashgill unconformity widely postulated in North Wales, but which is not pervasive in south and central Wales (Davies et al.1997, p.23).

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