The Swellies (Pwll Ceris), lying within Nelson’s gaze between the Britannia and Menai Suspension Bridges, is a choke point where strong tides of 5-8kts run, making it a significant navigational obstacle, particularly in the days of sail. This rock-strewn reach, eight cables long, was formed by fault line activity, glaciation and rising sea levels, which, about 5,000 years ago, finally inundated the strait, separating Anglesey from mainland Wales.
In 1693, Captain Grenvile Collins wrote in his Coasting Pilot “...the rocks at the Swilly are dangerous, the passage narrow, and the tydes very strong; and be sure to pass the Swilly at a slack tyde.”
Building the Telford’s Menai Suspension Bridge and Stephenson’s Britannia Bridge across the strait replaced the passenger ferry, the fording of cattle across the strait and provided a rail link to the port of Holyhead. A suitable air draft allowed continued navigation for sailing vessels of the day and the work provided opportunities to remedy some of the longstanding navigation problems.
Between 1824 and 1854, the engineers directed clearance work to reduce, by blasting, the significant hazards in the Swellies. These works cleared the south shore and lowered many of the rocks to low water springs (See chartlet). The Swelly Rock lost 160ft of its
