Menai Strait

• 1. Introduction

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  The Menai Strait is a narrow channel of water that has swallowed ships, inspired opera, and sheltered creatures found nowhere else on earth. Running for about 25 km between the island of Anglesey and the Welsh mainland of Gwynedd, it connects Caernarfon Bay to the south-west with Conwy Bay to the north-east. Both bays open into the Irish Sea, and the strait sits between them like a corridor through which the ocean does exactly as it pleases.

  At its narrowest, between Fort Belan and Abermenai Point, the channel squeezes to 400 m wide. At its broadest, between Puffin Island and Penmaenmawr, it stretches to 7.5 km. That range in width is not just a geographical curiosity. It shapes the tides, the currents, and the fate of anyone who ventures into the water at the wrong moment.

  Two great bridges now span the strait. Thomas Telford completed the first in 1826; Robert Stephenson finished the second in 1850. A third was proposed, debated for years, and then cancelled in February 2023. Between the bridges lies the most dangerous stretch of all, a tidal zone the Welsh call Pwll Ceris, where rocks lurk just below the surface. And scattered across the strait are islands, fish weirs, and a church on a small island that has stood here for centuries.

  How did this channel form? What battles were fought across its waters? And why do scientists from Bangor University consider it one of the most ecologically important stretches of water in Britain? Those are the questions this documentary sets out to answer.

• 2. Ice, Rock, and Rising Seas

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  Around 10,000 BC, as the last ice age ended and global ocean levels rose, the deepest of a series of glacially carved hollows in this part of Wales flooded with seawater. That flooding created the Menai Strait as it exists today.

  The channel follows a line of geological weakness known as the Menai Strait fault system. During successive Pleistocene glaciations, enormous ice sheets moved from northeast to southwest across Anglesey and the neighbouring mainland of Gwynedd. The underlying rock happens to run in the same direction, so the ice was able to scour the bedrock with particular efficiency. The result was a set of linear hollows cut into the rock, parallel ridges and troughs running in the same north-east to south-west orientation.

  The deepest of those hollows became the channel that separates Anglesey from the Welsh mainland today. At its deepest, the water reaches 15 m. The name itself carries a memory of the landscape: Menai derives from the Welsh main-aw or main-wy, meaning narrow water. The Welsh understood what the geology had produced long before geologists gave it a name.

  That ancient bedrock now lies beneath a water column that, in places, sees currents exceeding 7 knots. The very conditions that make the strait dangerous also make it remarkable, and the fish weirs built along both banks and on several of the islands turned that natural productivity into a food source for many centuries. Eight of those weirs are now scheduled monuments.

• 3. Battle, Piety, and Plunder

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  Echmarcach mac Ragnaill, an 11th century Norse-Gael ruler, came to Wales to plunder. According to the Heimskringla, he arrived with a Viking companion named Guttorm Gunnhildsson. The two men quarrelled over the spoils of their raid and settled the dispute in a battle at the Menai Strait. Guttorm won by praying to Saint Olaf, and Echmarcach was killed.

  A generation later, the strait became the setting for a very different kind of story. The Orkneyinga Saga recounts a Viking raid on Anglesey in the 12th century. Among the raiders was Magnus Erlendsson, Earl of Orkney, a man with a reputation for piety and gentleness. When the fighting began, Magnus refused to take part. He stayed on board his ship and sang psalms while his companions fought on the shore. That act of conscience is part of what made him a saint. He is known today as Saint Magnus.

  The story refused to stay in the medieval chronicles. In 1973, the Orcadian author George Mackay Brown retold it in his novel Magnus. Four years later, the composer Peter Maxwell Davies turned it into an opera called The Martyrdom of St Magnus. The first of the opera's nine parts is titled The Battle of Menai Strait, preserving the name of the place where a Norse earl chose psalms over violence.

  For centuries after those battles, the strait became a commercial waterway rather than a military one. From the 1890s until 1963, the pleasure steamers of the Liverpool and North Wales Steamship Company ran their main route from Liverpool and Llandudno along the strait and around Anglesey. After the company went into voluntary liquidation in 1962, P and A Campbell took over services for a time.

• 4. The Bridges That Changed Everything

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  Thomas Telford completed the Menai Suspension Bridge in 1826, and at 417 m long and 30 m high, it was the first bridge ever to cross the strait. It carries the A5 road, which runs from London to Holyhead on Holy Island. The bridge is grade one listed and has been put forward as a candidate for UNESCO World Heritage status.

  Robert Stephenson finished the Britannia Bridge in 1850. At 461 m long and 40 m tall, it was built as a rail bridge connecting Anglesey to the Welsh mainland and originally took the form of a tubular railway bridge. It carries the North Wales Coast Line between Holyhead and Crewe. That original structure survived for more than a century before fire severely damaged it in 1970. Between 1970 and 1972 engineers rebuilt and adapted it as a truss arch bridge, and the rebuilt version also carries the A55, a dual carriageway connecting Chester to Holyhead. The Britannia Bridge is grade two listed and is the more commonly used of the two crossings.

  For decades, a third crossing was discussed. Government proposals for a Third Menai Crossing emerged as far back as 2007, aimed at reducing congestion on the two existing bridges. On the 14th of February 2023, however, the Welsh Government announced the project would not go ahead. The government cited three reasons: a policy commitment to reducing car use, the environmental impact of the crossing, and a description of the scheme as a blot on the landscape. Financing difficulties were later added as a further reason. Lee Waters, deputy minister for climate change, left the door open, suggesting the crossing could be revisited as part of a broader infrastructure review for North Wales.

• 5. Tides That Defy Intuition

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  Slack water between the two bridges tends to occur approximately one hour before high tide or low tide, a pattern that surprises visitors who expect the water to be still at the conventional tidal turning points. The explanation lies in the geometry of the strait itself.

  A rising tide approaches from the south-west, pushing water north-eastwards along the channel. But the same tide also wraps around Anglesey. After a few hours, it begins to enter the strait from the Beaumaris end, flowing in the opposite direction. By the time this south-westerly flow from the north-east starts to strengthen, the tidal current from the Caernarfon end is already weakening. The water level continues to rise even as the direction of flow reverses. A falling tide produces the same sequence in reverse. The result is that two ends of the same channel can be moving in opposite directions at the same time.

  The most dangerous product of this system is the stretch called the Swellies, in Welsh Pwll Ceris, lying between the two bridges. Rocks close to the surface create overfalls and localised whirlpools. In 1953, the training ship HMS Conway was lost there. In theory, the Swellies can be forded on foot at low water during spring tides, when the depth may drop to less than half a metre. In practice, the current running at those moments reaches approximately 4.8 knots, making any crossing extremely difficult.

  The Caernarfon end presents its own hazard. Sand banks at Caernarfon bar shift frequently, creating conditions that disorient even experienced navigators.

• 6. Ecology at the Edge

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  Because the strait's tidal conditions are so unusual, and because its sheltered position keeps wave heights very low, it hosts a benthic ecology that scientists describe as unique and diverse. The channel's unusual current regime and depth profile produce an environment rich in sponges.

  This singular ecology was a key reason why Bangor University established its School of Ocean Sciences at Menai Bridge. The strait holds official status as a special area of conservation with marine components and is also a proposed Marine Nature Reserve. Few stretches of British coastal water carry that combination of designations.

  On land, the same unusual conditions have produced a comparable rarity. The banks of the Menai Strait are home to the Menai Whitebeam, a critically endangered plant classified as a species of Sorbus found only in this part of North Wales. The total population consists of about 30 plants, most of them thought to be mature. A species with a global range measured in dozens of individuals, all clustered along one strait in Wales, represents one of the most concentrated cases of botanical rarity in Britain.

  The land designations along the banks include several Sites of Special Scientific Interest: Glannau Porthaethwy, the ivy-oak-ash woodland along the southern shore known as Coedydd Afon Menai, and Lavan Sands, in Welsh Traeth Lafan. Much of the Anglesey land at the eastern end of the strait is also designated as an area of outstanding natural beauty. Church Island, known in Welsh as Ynys Tysilio, sits within the strait itself and holds St Tysilio's Church, a presence on these waters that predates every bridge, every battle record, and every scientific survey made here.