Pontcysyllte Aqueduct

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The Aqueduct, view from the ground

The River Dee runs beneath

A view of the ground below from the aqueduct

The Pontcysyllte Aqueduct pronounced [ˌpɔntkəˈsʌɬtɛ]) is a navigable aqueduct that carries the Llangollen Canal over the valley of the River Dee, between the villages of Trevor and Froncysyllte, Wrexham in north east Wales. Completed in 1805, it is both the longest and highest aqueduct in Britain, and is a Grade I Listed Building.^[1]

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, after which the joints were sealed with lead. Then it was left for six months with water inside to see if the trough 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 over the trough, which is the full width of the aqueduct, so that narrowboats are able to move more freely 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.

There used to be an accessible small handle in a recess on the footpath in the middle of the centre span^{[citation needed]}, the pulling of which would cause a cascade of water to tumble into the River Dee below. This cascade can still be seen every few years when the aqueduct is emptied for maintenance.

The aqueduct was suggested as a contender for World Heritage status in 2005, its 200th anniversary year,^[2] and formally nominated in 2006.^[3]