Historical documents provide a record of a great storm in 1331. In southwest Anglesey, a major storm is reported to have caused major upheaval and movement of sand. A village, near what is now Newborough, was buried in sand and abandoned.

At Aberffraw, it caused sand to overwhelm land previously used for agriculture. The sand was blown from the beach up to 3 km inland where it has blocked a small river, resulting in the formation of a shallow lake, now known as Llyn Coron. The shape and extent of Aberffraw Bay was considerable altered and reduced, affecting the navigation of ships and boats to the port at Aberffraw.
There is good evidence for the existence of extensive dunes in southwest Anglesey far earlier than the earliest written records of 1331. So dunes must have existed around Aberffraw before this date but the Great Storm made a dramatic difference to their location in the bay. Dune systems are normally dynamic. They undergo periods of erosion and sand accumulation. They move year by year; they are constantly changing in response to the weather. A storm of epic proportions could change them overnight.

Today, sand covers about 3 km², with three rows of dunes above a sand sheet at least 8 m thick. The sand at the edge of Llyn Coron shows evidence of deposition around the date of the Great Storm. There is also evidence of further sand deposits around Llyn Coron in about 1480 AD and again in1680 AD. This might indicate further storms and associated dune movement, which would have altered the flow the Afon Ffraw and its estuary even more.