Thomas Telford's causeway between Valley and Holy Island was the last stage of his great road between London and Holyhead (A5) (NPRN 402349). The Stanley Embankment is 1.2km long, some 35m wide at the base, and up to 4.8m high. It crosses Beddmanarch Bay and was opened in 1823. There is a quay built into the north side (NPRN 402350). The causway was built in one year by Gill and Hodges and was opened in 1823.

It is similar in design to Telford's earlier causeway, The Mound at Dornoch in north east Scotland, which was constructed between 1814-16 to carry the Great North Road (A9) across Loch Fleet. Both embankments still carry the A5 and A9 respectively and both were later widened to carry railways.

Source: David Gwyn & Merfyn Williams (1996) `A Guide to the Industrial Archaeology of North West Wales?. Association for Industrial Archaeology

B.A.Malaws, RCAHMW, 20 December 2007.