The stratigraphy of the Bala district, Merionethshire

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Abstract
About 45 square miles of Ordovician and Silurian rocks exposed around the town of Bala have been mapped and their brachiopodtrilobite faunas studied in order to revise the stratigraphy of the Bala Group and effect a more exact correlation with other standard Ordovician sections. The succession established by Elles (1922 a ) has been emended to include a quartzite more or less contemporaneous with the Hirnant Limestone and an ash at the revised base of the Glyn Gower Siltstones, and to allow for hitherto unsuspected complications due to unconformities and faulting. The proposed changes in stratigraphical terminology have been framed in accordance with the American Code of Stratigraphical Nomenclature. The infra-Rhiwlas Limestone unconformity, first indicated by Bancroft (1928 c ) is now known to have involved great variation in the depth of erosion, especially northwards. Sudden localized changes in oversteps have been attributed to pre-Rhiwlas Limestone faulting. An outlier of high Ashgillian and Llandovery rocks west of their main outcrop has revealed the presence of another unconformity within the higher part of the Ordovician succession which may also be due to contemporaneous fault slip. These breaks suggest that the use of ‘Bala Series’ as a standard subsystemic division of the Ordovician should be discontinued although the term ‘Bala Group’, divided into ‘Lower’ and ‘Upper’ at the main unconformity, may be retained for Upper Ordovician successions of the region. The discovery of new fossil localities, particularly in the Nant Hir Mudstones and Glyn Gower Siltstones, the recognition of assemblage zones in the Allt Ddu Mudstones, and a faunal succession in the Gelli-grin Calcareous Ashes permit correlation with the type-Caradocian although it has not been possible to locate the Caradocian stage boundaries precisely. The Lower Bala Group can be dated as Costonian to Longvillian in age, the base of the Lower Longvillian being more or less coincident with the base of the Gelli-grin Calcareous Ashes. The Rhiwlas Limestone, the basal member of the Upper Bala Group, contains a rich trilobite fauna that compares closely with assemblages from other British strata containing Phillipsinella parabola and is regarded as Middle Ashgillian in age. The Foel y Ddinas Mudstones, emended to include the Hirnant Limestone as a member, contain the youngest Ordovician shelly fauna known in Britain.