The Menai Strait is a narrow stretch of shallow tidal water about 16 miles long, separating the island of Anglesey from the mainland of Wales.
The channel is a result of glacial erosion of the bedrock along the Menai Strait Fault System. During a series of Pleistocene glaciations, a succession of ice-sheets moved from northeast to southwest across Anglesey, scouring the underlying rock. A series of linear bedrock hollows were formed across the region. The deepest of these were flooded by the sea as world ocean levels rose at the end of the last ice age.
The differential tides at the two ends of the strait cause very strong currents to flow in both directions through the strait which create dangerous conditions. A dangerous area of the strait is known as the Swellies, located between the two bridges. Rocks near the surface cause over-falls and local whirlpools. The training ship HMS Conway was lost here in 1953.
The strait has two bridges, the Menai Suspension Bridge, carrying the A5 road and Robert Stephenson's 1850 Britannia Tubular Bridge. The Britannia bridge carried rail traffic in two wrought-iron rectangular box spans, but after a disastrous fire in 1970, only the limestone pillars remained, it was rebuilt as a steel box girder bridge, to carry both rail and the A55 road.
Between the two bridge crossings there is a small island in the middle of the strait, Ynys Gorad Goch, on which are built a house and outbuildings and around which are the significant remains of fish traps, no longer used. The tides carry large quantities of fish, and the construction of Fish weirs on both banks and on several of the islands, helped make the Strait an important source of fish for many centuries. Eight of the numerous Menai Strait fish weirs are now scheduled monuments.