The Llandovery and Associated Rocks of the Neighbourhood of Corwen

Abstract

I. Introduction. It was for many years a doubtful point whether true Llandovery rocks were represented in the more northern part of Wales, or whether the Tarannon Shales here rested directly on the Bala Beds. Thus, according to the first edition (1866) of vol. iii. of the Geological Survey Memoirs (p. 205 et seq .), no Llandovery rocks are known between Conway and the country east of Bala Lake ; and the Lower Llandovery first appears near Cefn-bwlan. But it is at the same time noted in the appendix (p. 275) that Lower Llandovery fossils occur on Cyrn-y-brain (north of Llangollen) and, doubtfully, near Pwllheli. In a paper published in 1877, however, Prof. T. Mc K. Hughes described a series of grits at Corwen which rest upon Bala Beds, and are overlain by pale beds passing up into the ‘Pale Slates’ (Tarannon Shales) of the Survey. To these grits he gave the name of Corwen Grits. Near Corwen itself he found in them no fossil except Favosites alveolaris (doubtful); but a similar grit occurs in the same stratigraphical position in various other localities, and on Cyrn-y-brain he obtained, in fragments of what seems to be the same grit, Petraia subduplivata, P. crenulata, and Meristella crassa . Hence he concludes that the Corwen Grit is of Llandovery age, and that it forms the base of the Silurian in this area. Two years later Mr. T. Ruddy described a number of sections in the same district. Everywhere he finds that the Tarannon Shales are