The Menai Strait was created by glacial erosion along a line of weakness associated with the Menai Strait fault system. During a series of
Pleistocene glaciations (that lasted from about 2,580,000 to 11,700 years ago), a succession of ice-sheets moved from northeast to southwest across
Anglesey and neighbouring
Gwynedd, scouring the underlying rock and creating a series of linear bedrock hollows. The deepest of these channels eventually became flooded by the sea as the ice sheets receded, forming the Menai Strait. As Anglesey has been an island throughout recorded human history, the only way to reach it was by crossing the strait. However, this has always been a dangerous endeavour because there are four strong tidal flows each day generated by the twice daily tides. These flow in both directions through the strait, creating strong currents and whirlpools. Despite the dangers, ferries operated all along the Menai Strait, carrying passengers and goods between the island and the mainland. In 1785, a boat carrying 55 people ran aground at the southern end of the Menai Strait in a strong gale and began to sink. The stricken vessel sank before a rescue boat from
Caernarfon could reach it, and only one person survived. Additionally, the main source of income on Anglesey was from the sale of cattle, and to move them to the markets of the mainland, including
London, they had to be driven into the water and encouraged to swim across the Strait. This often resulted in the loss of valuable animals. In 1800, Ireland joined Great Britain in the
Act of Union. This led rapidly to an increase in people travelling between London and Holyhead en route to Dublin. In 1815, the
British Parliament passed an act of Parliament, the
Holyhead Roads Act 1815 (
55 Geo. 3. c. 152), to build the
Holyhead Road with responsibility for the project given to civil engineer
Thomas Telford. Despite some difficult geographical obstacles to overcome (e.g.
Snowdonia and the Menai Strait), the route was chosen because Holyhead was the principal port for ferries to
Dublin as it was the closest point to Ireland. After Telford had completed a survey of the route from London to Holyhead, he proposed that the best option was to build a bridge over the Menai Strait from a point near
Bangor on the mainland to the village of Porthaethwy (now commonly known as
Menai Bridge) on Anglesey. The site for the bridge was chosen because it had tall banks that would be high enough to allow
sailing ships to pass underneath. Telford proposed that a suspension bridge would be the best option because it would have a span wide enough to cross the fast flowing waters of the Strait at this point. His recommendation was accepted by Parliament, which authorised the bridge in the
Roads Between London and Holyhead Act 1819 (
59 Geo. 3. c. 48). ==Construction==