Around 20,000 years ago, Anglesey was completely submerged by ice. Two ice sheets from different sources were involved. The Snowdonian mountains were the source of ice streams that moved broadly northwards towards Anglesey, while the massive Irish Sea ice sheet, fed by glaciers from Scotland, Ireland and Cumbria, moved onto the island from the north.

The Irish Sea ice stream was dominant, and travelled north-east to south-west across the island, broadly in keeping with its NE-SW-trending, structurally controlled rock ridges.

The Welsh and Irish Sea ice streams met in the region of the present-day Menai Strait and produced a confluent south-westward flow. (source GeoMon).

In late glacial times the north-south valley was enlarged as an overflow channel for an ice-dammed lake to the northeast. Postglacial subsidence created a continuous marine channel from the three sections.