The rocks which today make up Snowdon and its neighbouring mountains were formed in the Ordovician Period. At that time, most of modern-day Wales was near the edge of Avalonia, submerged beneath the ancient Iapetus Ocean. In the Soudleyan stage ( 457) of the Caradoc age, a volcanic caldera formed, and produced ash flows of rhyolitic tuff, which formed deposits up to thick. The current summit is near the northern edge of the ancient caldera; the caldera's full extent is unclear, but it extended as far as the summit of Moel Hebog in the south-west.