"The second wonder is in that place the mountain/large stone that circles by three turns in the year / in a circuit"1,2

There can be little doubt that this is a folklore wonder, rather than a physical phenomena. If it is a piece of folklore it is much more likely that it refers to a large stone that rotates than a mountain. Moving stones are a wellknown folk motif3, and this would be the earliest British example. While the wonder doesn't seem to correspond to any current folklore in Anglesey, there is one close contender: the stone Lleidr Tyfrydog or Carreg Y Lleidr ["Tyfrydog's Thief", or the "Thief's Stone"] at Clorach. This stone is meant to run three times around its field on one night of the year; this matches the description if the translation is taken to refer to a stone which moves three turns around a circle once a year, rather than a mountain that slowly rotates every four months.

You can read more on the Myth of Lleidr Tyfrydog...