The Menai Strait formed during the Pleistocene epoch when ice sheets moved from northeast to southwest across Anglesey and neighbouring Gwynedd. These massive glaciers scoured the underlying bedrock along a line of weakness known as the Menai Strait fault system. The grain of the rock ran in the same direction as the movement of the ice, creating deep linear hollows. As world ocean levels rose at the end of the last ice age around 10,000 BC, these hollows flooded to become the present day channel. This geological process shaped the strait into its current form over thousands of years before human history began.
Viking Raids And Medieval Battles
According to Heimskringla, the 11th century Norse-Gael ruler Echmarcach mac Ragnaill plundered in Wales with his friend, the Viking Guttorm Gunnhildsson. They started quarreling over the plunder and fought a battle at the Menai Strait. Guttorm won the battle by praying to Saint Olaf and Echmarcach was killed. In the 12th century, a later Viking raid and battle in the Menai Strait are recounted in the Orkneyinga Saga as playing an important role in the life of Magnus Erlendsson, Earl of Orkney. He had a reputation for piety and gentleness. Refusing to fight in the raid on Anglesey, he stayed on board his ship, singing psalms. This incident is recounted at length in the 1973 novel Magnus by Orcadian author George Mackay Brown, and in the 1977 opera, The Martyrdom of St Magnus by Peter Maxwell Davies.Tidal Currents And Swellies Hazards
The differential tides at the two ends of the strait cause very strong currents which create dangerous conditions. One of the most hazardous areas is the Swellies between the two bridges where rocks near the surface cause over-falls and local whirlpools. This was the site of the loss of the training ship HMS Conway in 1953. Entering the strait at the Caernarfon end is also hazardous because of the frequently shifting sand banks that make up Caernarfon bar. A rising tide approaches from the south-west causing the water in the strait to flow north-eastwards as the level rises. The tide also flows around Anglesey until after a few hours it starts to flow into the strait in a south-westerly direction from Beaumaris. By this time the tidal flow from the Caernarfon end is weakening and the tide continues to rise in height but the direction of tidal flow is reversed.