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A thick succession of Middle Caradoc sediments (Glanrafon Beds) with occasional volcanic horizons is described. Faunas found within the succession prove a Soudleyan to Lower Longvillian age for these sediments. Detailed mapping has shown the presence of an unconformity between the Glanrafon Beds and the overlying Lower Rhyolite Tuff of the Snowdon Volcanic Series.A new fauna from the sediments within the Capel Curig Volcanic Series proves a Soudleyan (?Lower) age for this period of vulcanicity. INTRODUCTIONA thick succession of sediments with thin, impersistent volcanic horizons occurs stratigraphically between the Ordovician Capel Curig and Snowdon Volcanic Series, in Caernarvonshire, North Wales. They have been called the Lower and Upper Glanrafon Beds in the Moel Hebog area by Shackleton (1959) and subdivided into the Gwastadnant Grits underlain by Glanrafon Slates to the northeast of Snowdon by H. Williams (1927). On the basis of the contained faunas, these sediments have been assigned to various zones of the Caradoc by some workers and by D. A. B. Davies (1936) to the Llandeilian. The authors decided to reinvestigate the Glanrafon sediments of the Llyn Cowlyd area with a view to establish more accurately the age of these sediments and to define more closely the time of onset of the Snowdon vulcanicity in this part of eastern Snowdonia. Faunas from sediments within the Capel Curig Volcanic Series were also collected and examined to determine the age of this period of vulcanicity in relation to the other Ordovician volcanic episodes of North Wales.The area studied extends about two square miles around Llyn Cowlyd, near Capel Curig, Caernarvonshire, North Wales and lies to the north of the A5 (BangorBetws-y-coed) road (Fig. 1). It is covered by sheet SH 76 SW 6 ins to 1 mile Provisional Edition of the Ordnance Survey. The ground lies to the northeast of the main mass of Snowdon volcanic rocks and sediments and is principally composed of sandstones and fine grained siltstones with occasional rhyolite plugs and thin impersistent bands of laharic breccias, overlain by volcanic rocks of the Snowdon Volcanic Series.The topography of the area rises from 1,200 ft to 2,622 ft and the extent of exposed ground varies from 65%-70% above 1,500 ft to 5%-25% below this level. Much of the lower slopes of the valley enclosing Llyn Cowlyd are covered by solifluction deposits and scree. Glacial drift blankets most of the ground to the south of Bwlch Cowlyd and is covered by peat from 1-3 ft thick, but elsewhere in the area glacial drift is virtually absent.The area was first mapped by Ramsay and Aveline and described ifi the Memoir of the Geological Survey (1866) with the accompanying Geological Survey map sheet 78 SE (Old Series). Seventy years later Dakies included the Llyn Cowlyd area in his paper on 'Ordovician rocks of the Trefriw district ' (1936). In 1946, A. Lamont publishing on B. B. Bancroft's geological work of the Capel Curig district referred to the present area and included a sketch-map showing fossil localities an...

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