Crafnant and Snowdon Volcanic groups - Capel Curig and Betws-y-Coed. Description of 1:25 000 sheet SH 75
|From: Howells, M. F., Francis, E. H., Leveridge, B. E. and Evans, C. D. R. 1978 Capel Curig and Betws-y-Coed. Description of 1:25 000 sheet SH 75 Classical areas of British geology, Institute of Geological Sciences. (London: Her Majesty's Stationery Office.)
Chapter 3 Crafnant and Snowdon Volcanic groups
Late Caradoc volcanism produced large volumes of eruptive rocks which originally covered the whole district. Complete sequences are now found, however, only in the Dolwyddelan Syncline and in the area north of the Mon Llugwy, between Capel Curig and Betws-y-Coed, though in the ground between those areas lower members of the sequence are preserved in small synclinal outliers (Figure 9).
The Dolwyddelan rocks were correlated by Williams and Bulman (1931) with the succession in Central Snowdonia (Williams, 1927) and named the Snowdon Volcanic 'Suite' (now Group), comprising three divisions, namely Lower Rhyolite-Tuffs, Bedded Pyroclastic 'Series' and Upper Rhyolite-Tuffs. The rocks north of the Llugwy were similarly correlated with the Snowdon volcanics by Davies (1936), who applied the local term Crafnant Volcanic 'Series'. Both nomenclatures are retained in this account, modified only to meet modern requirements of stratigraphical terminology. In addition the Crafnant Volcanic Group has been divided into Lower, Middle and Upper formations (Howells and others, 1971). The lower formation has been correlated in some detail with the lower formation of Dolwyddelan ((Figure 18); Howells and others, 1973). The middle and upper formations of the two areas can also be broadly equated, though they show more lateral variation in lithology than the lower formation. In particular, the essentially basic nature of the Bedded Pyroclastic Formation in Central Snowdonia becomes markedly less pronounced when traced from west to east within the Dolwyddelan Syncline, while the equivalent Middle Crafnant Volcanic Formation is almost entirely acid in composition.
Soudleyan faunas have been recorded at or near the base of the group in Central Snowdonia (Harper in Shackleton, 1959; Williams and Harper in Beavon, 1963), whereas in the Capel Curig and Betws-y-Coed district the Soudleyan–Longvillian boundary is firmly drawn within the sandstone-tuffite assemblage some distance below the base of the group (p. 3). It is uncertain whether this implies an earlier start of the main volcanism in Central Snowdonia or anomalies in the preservation and collection of specimens. The Crafnant–Snowdon Volcanic Group yielded few fossils during the recent survey, but its mid-Caradoc age is established by the Diplograptus multidens Zone fauna contained in the Middle Crafnant Volcame Formation (p. 70), and in the black slates overlying the Group at Dolwyddelan and Betws-y-Coed.
Black slates occur below the uppermost formation of both the Snowdon and Crafnant Volcanic groups.
In the following account the two areas are described separately under local formational headings.