Craig-y-Garn
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Geology & landscape
Geological Period: Ordovician · Rock Type: Rhyolite
Landscape processes: Volcanic / Igneous
Description
Description This site is of special scientific interest because it provides an internationally important section through one of the main eruptive centres belonging to the 2nd Eruptive Cycle of mid-Caradoc (Upper Ordovician) volcanic activity in Snowdonia. With a minimum exposed thickness of approximately 700 m, the Pitts Head Tuff Formation exposed on Craig-y-garn represents one of the thickest known accumulations of ash-flow tuffs (ignimbrites) related to an individual volcanic centre in Britain. The site is located in a mountainous tract of land approximately 8 km north-west of Porthmadog and 1 km east of the village of Garndolbenmaen. Craig-y-garn provides a critical section through a subaerially emplaced sequence of vitroclastic ash-flow tuffs belonging to the Pitts Head Tuff Formation, a unit previously referred to as the Llwyd Mawr Ignimbrite. The ash-flow tuffs were erupted within a subsiding volcano-tectonic depression or caldera where, based on textural evidence and joint geometry, they cooled as a single unit. The latter stages of caldera evolution were marked by the forceful emplacement of rhyolite domes and sills along the caldera margin and within feeder pipes or vents in the centre of the structure. The intrusive bodies of rhyolite are geochemically and petrographically similar to the ash-flow deposits and may represent a late-stage, degassed equivalent of the tuff magma. Remarks: The site lies within Snowdonia National Park.