The Stanley Embankment, Holyhead
The historic A5 road crosses the water between Anglesey and Holy Island on this causeway. Its construction resulted in an unusual marine environment, known as the Inland Sea, forming to the south.
Previously road traffic and pedestrians had to use the Four Mile Bridge, over the narrowest stretch of water, but engineer Thomas Telford’s task was to provide a direct and speedy route for mail coaches between London and Holyhead. To cut out a detour of about 6km, he designed an expensive embankment across the Stanley Sands, named after one of Holyhead’s most influential families. The embankment is 1.2km long. It was built in just one year and opened in 1823.
Road users paid a toll at the western end of the embankment. The toll house still stands but was moved a short distance in the 1960s.
The embankment was widened in the 1840s for the Chester & Holyhead Railway’s tracks. The railway is behind the high stone wall to the south of the road. Beyond the railway is a second embankment, a rock causeway constructed at the start of the 21st century for the A55 dual-carriageway.
Culverts under the embankments allow seawater to flow from one side to the other. The Four Mile Bridge forms a similar constriction 2km further south. The result is that the water between the structures has a narrower tidal range than the sea beyond. The Holy Island Strait, commonly called the Inland Sea, is also fed by freshwater from small streams.
The enclosed nature and interesting currents make the Inland Sea a popular area for boating and kayaking.