Yr Arddu is a 389m high mountain peak in Snowdonia, Wales (Not to be confused with the 589 metre Yr Arddu to the east of Snowdon). It lies approximately 2.5 km south east of the village of Beddgelert and just over 1 km from the rather more famous mountain Cnicht. It is a rugged hill with widespread rock outcrops and cliffs. A small lake, Llyn yr Arddu, is located on the flanks of the mountain. The craggy nature of the summit is due to the presence of lavas and tuffs (volcanic ash deposits) dating from the Ordovician period when Snowdonia was characterised by numerous volcanic eruptions; Yr Arddu is a particularly good location to see the record of Ordovician volcanism.