The legend of Seithennin : drowned lands around the Anglesey coastline
All around the Welsh coastline, including Anglesey, there are tales of drowned lands. Seithennin was the unlucky watchman responsible for checking the floodgates were closed whenever the tide came in. One night, after imbibing too much drink so the legend goes, he fell asleep and forgot to check the gates. That night, the tide came in, drowned all the lands and never went out again [watch BBC Cymru video]. Around the coast, tide bells have been placed as an art installation commemorating the legend. There is one in Cemaes.
Geologically, a more prosaic explanation exists; after the last ice age, sea level rose dramatically from about 100 metres below the present sea level as all the ice caps and glaciers melted, returning the water to the sea. Evidence can be found around the coastline in the form of submerged forests, peat beds and soil. Trearddur beach is the best place to see tree stumps in situ and fallen logs, but remnants can also be found in Porth Diana, Penrhos beach, Lleiniog and Lligwy.
By far the best examples are on the beach at Borth, just north of Aberystwyth