Yr Arddu
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Geology & landscape
Geological Feature: Fissure · Geological Period: Ordovician · Rock Type: Breccia
Landscape processes: Volcanic / Igneous
Description
Description This site is of special scientific interest because it provides a magnificent section through a fissure-controlled eruptive centre of mid-Caradoc (Upper Ordovician) age. A thick sequence of ash-flow deposits (the Yr Arddu Tuffs) was later intruded by rhyolitic domes and transected by an explosive breccia dyke. The site is located in a mountainous tract of land lying between the Afon Nanmor and Afon Dylif, approximately 4 km south-east of Beddgelert. A thick (c. 180 m) sequence of vitroclastic ash-flow tuffs (the Yr Arddu Tuffs), which are of mid-Caradoc (Longvillian Stage) age, are superbly exposed on and around the flanks of the Yr Arddu massif. This sequence forms the basal unit of the Lower Rhyolitic Tuff Formation (Snowdon Volcanic Group) and illustrates the dominantly acidic nature of this early phase of extrusive volcanic activity. Yr Arddu also exhibits a possible fissure-vent filling in the form of a breccia dyke, and rhyolitic intrusions with spectacular, folded flow banding. This site, together with other members of the Ordovician Igneous GCR network in central Snowdonia, demonstrates the character of igneous activity in one of the classic volcanic regions of the British Isles. Remarks: The site lies within Snowdonia National Park.