The Swellies

Sun 22nd April 2018

The Swellies are a section of water that lies between the Britannia Bridge and the Menai Suspension Bridge, in the Menai Strait.

Deep fissures and large frock formations cause the waters here to bubble and swirl at certain tides. The Swellies is the most volatile section on the whole Strait. It is narrow and shallow with rough waters.

The tide at the Swellies

Every 6hrs approximately 30 million cubic metres of water passes through the Strait. That’s equivalent to over 20 Millennium Stadiums full of water – four times a day.

Every tide water rushes into the Strait from Penmon Point, near Puffin Island, and Abermenai Point, at the end of Llanddwyn beach. These tides meet at Swellies Rock.

The rocks are marked with a Southerly Cardinal Mark, this means you are to pass to the south of this marker. Locals sometimes call this mark ‘Swellies Pole’. Learn more about Cardinal Marks at

The Swellies Cardinal Mark South, Anglesey in the Menai Strait

You travel through the Swellies and past Swellies Pole on a Bridges and Swellies adventure boat tour.