Pontcysyllte Aqueduct

Pontcysyllte Aqueduct

The Pontcysyllte Aqueduct is a navigable aqueduct that carries the Llangollen Canal over the valley of the River Dee, between the villages of Trevor and Froncysyllte, in Wrexham in north east Wales. Completed in 1805, it is the longest and highest aqueduct in Britain, and is a Grade I Listed Building.


The aqueduct, built by Thomas Telford and William Jessop, is 1,007 ft (307 m) long, 11 ft (3.4 m) wide and 5.25 ft (1.60 m) deep. It consists of a cast iron trough supported 126 ft (38 m) above the river by nineteen hollow masonry piers (pillars). Each span is 53 ft (16 m) wide. Many people were sceptical, but Telford was confident: he had built at least one previous cast iron trough aqueduct (the Longdon-on-Tern aqueduct on the Shrewsbury Canal, still visible in the middle of a field, though the canal was abandoned years ago).

The mortar used comprised lime, water and ox blood. The iron castings were produced at the Plaskynaston Foundry, and each casting dovetails into the next. To caulk the joints, Welsh flannel was dipped in boiling sugar[clarification needed], after which the joints were sealed with lead. Then it was left for six months with water inside to check it was watertight.

Part of what was originally called the Ellesmere Canal, it was one of the first major feats of civil engineering undertaken by leading civil engineer Thomas Telford (supervised by the more experienced canal engineer William Jessop). The iron was supplied by William Hazledine from his foundries at Shrewsbury and nearby Cefn Mawr. It was opened on 26 November 1805, having taken around ten years to design and build at a total cost of £47,000.

The towpath is cantilevered from the side of the trough, which is the full width of the aqueduct. This arrangement allows the trough to be the maximum width and thus the displaced water from ahead of the boat can more easily flow past the boat, ensuring that narrowboats are able to move as freely as possible through the water. Walkers are protected by railings on the outside edge of the towpath, but the holes to fit railings on the other side of the aqueduct were never used. As the edge of the trough is only about 6 inches (15 cm) above the water level, and therefore below the deck of a narrowboat, the boat steerer has nothing between them and the sheer drop.

A plug exists in the centre of the of aqueduct. to allow for draining canal water into the River Dee below for maintenance.

World Heritage application

The Aqueduct and surrounding lands were submitted to the tentative list of properties being considered for UNESCO World Heritage Site status in 1999. The aqueduct was suggested as a contender in 2005—its 200th anniversary year—and it was formally announced in 2006 that a larger proposal, covering a section of the canal from the Aqueduct to Horseshoe Falls would be the United Kingdom's 2008 nomination.

The length of canal including the main Pontcysyllte Aqueduct structure were visited by assessors from UNESCO during October 2008, to analysis and confirm the site management and authenticity. The final announcement of success or failure is scheduled for July 2009.