Discussion

Abstract

Written contribution from Sir W illiam P ugh : It is over forty years since G. L. Elles described the Bala area and nearly forty years since B. B. Bancroft published work on its fossils. It is thus satisfactory to have an up-to-date account of the Bala rocks in which special attention has evidently been given to stratigraphy and palaeontology. With certain emendations, the general succession given by Elles stands but greater precision has been given to the divisions and the fuller knowledge gained of the brachiopod–trilobite faunas will be especially useful in correlation. The age of the Hirnant Beds, long disputed, is confirmed as Ordovician and the authors have grouped these beds with the Foel y Ddinas Mudstones as the highest division of the Upper Bala. For many miles south of Bala, the Foel y Ddinas Mudstones are succeeded by a distinctive group of shales, which are the local equivalents of the Hirnant Beds, but lateral changes in lithology presumably prevent separation of these two groups in the Bala area. The break between the Upper and Lower Bala was first recognized by Bancroft but the authors by their detailed mapping have revealed its magnitude. The Rhiwlas Limestone may rest not only on different levels in the Geili-grin Calcareous Ashes but also on the underlying Allt Ddu Mudstones, thus indicating the removal of a considerable thickness of Lower Bala before the deposition of the Upper Bala sediments. Many years ago, I mapped the Bala rocks for some thirty miles along the southern