The Pontcysyllte Aqueduct carries the Llangollen (Ellesmere) Canal 1007ft (307m) across the River Dee to the south of Trefor Basin. It is one of the tallest cast iron aqueducts in the world, rising 127ft (39m) above the river bed and was built by William Jessop and Thomas Telford between 1795 and 1805, at a cost of £47,018.

The aqueduct comprises of 18 stone piers, on which is carried a cast iron trough. The location of the quarry and construction yard on the north (Trefor) side of the river determined the sequence for the construction of the structure. The stone piers were raised in stages starting with the lower sections of the river piers and then moving south to north across the valley, between six and eight metres in height at a time. Five timber gangways were constructed to transport materials, possibly via rail, and to provide platforms from which to work. Each gangway was supported on two timber beams supported by the stonework of the piers and by diagonal braces held in cast iron shoes secured to the stonework. Evidence of these features survive in the stonework of each pier and help verify a series of contemporary paintings which show the aqueduct under construction. The stone piers themselves are tapered and are also hollow in their upper sections, to reduce their overall weight.

The upper, trough, section of the aqueduct is entirely of cast iron, produced at a specially commissioned forge at nearby Plas Kynaston. The trough was constructed in 19 sections and is formed from 2 inch thick plates bolted together along flanges, the joints made watertight with a mixture of flannel, white lead and iron borings. Each section was supported by four arch ribs, cast in three sections and bolted together with connecting plates. The ribs sat on springing plates near the top of each stone pier. In order to maintain flexibility, the trough merely sits on top of these ribs, stopped from lateral movement by a series of brackets and lugs and weighted down by the volume of water it contains. The original wooden towpath was replaced in 1831 with the current cantilevered design. Both were designed to enable a wider circulation of water underneath the towpath, preventing the overflow of water. Although the use of cast iron was revolutionary, enabling the spectacular achievement of constructing the tallest and longest navigable aqueduct of its time, the form of the upper section harked back to earlier traditions with the outer ribs designed to give the sense of solidity and the raked side plates imitating the voussoirs of a stone arch.

Louise Barker & Susan Fielding, RCAHMW, 21st October 2008.

The aqueduct was inscribed as a World Heritage Site by UNESCO on 27 June 2009.
B.A.Malaws, RCAHMW, 27 June 2009.

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The Pontcysyllte Aqueduct was constructed between 1796 and 1805 to carry the Ellesmere Canal over the River Dee, linking the Froncysyllte and Trefor villages situated on either side of the valley. It was designed and built by civil engineer Thomas Telford, acting as General Agent, and the more experienced consulting engineer William Jessop who undertook the site surveys and advised on best courses of action. The construction of Pontcysyllte Aqueduct cost £47,000. Standing 39 metres tall and 307 metres across, it is now the longest navigable aqueduct in Great Britain and the tallest in the world.

The aqueduct consists of eighteen stone piers that carry a cast iron trough which forms the canal. The piers are hollow and taper towards the top in an effort to reduce their weight and so allow for such a tall construction. For mortar, it is said, the engineers relied on a mixture of lime, water and ox blood. The cast iron trough is not fixed to the stone work, but merely sits on top of the series of sprung iron ribs which rise from each pier and is anchored by the weight of the water alone. The original towpath was replaced in 1831 by a cantilevered design that allowed the water displaced by passing boats to flow underneath it, reducing drag.

The aqueduct was constructed at the time of the emergence of modern tourism in Wales. Among the early continental visitors was the Archduke John of Austria who, with the end of the Napoleonic Wars, took the opportunity of travelling to Wales in 1816. Praising the beauty of the Dee valley in general, he then particularly recommended viewing the canal from below because it allowed a better appreciation of the intricate construction design. Other visitors were less interested in the engineering aspects of the aqueduct, but appreciated it more for its architectural beauty and harmony with the surrounding landscape. On 22 June 1830, the French landscape artist Alphonse Dousseau produced a faithful watercolour drawing of Pontcysyllte Aqueduct, complete with iron railing and set against the backdrop of the picturesque Dee valley at sunset and with a view of the ruins of Castell Dinas Bran in the far distance.

In 2009, Pontcysyllte Aqueduct was inscribed as World Heritage Site by UNESCO, recognising its innovative engineering design and great significance for the historical development of British waterways.

Record updated as part of the AHRC-funded project 'Journey to the Past: Wales in historic travel writing from France and Germany'.
R. Singer (Bangor University) and S. Fielding (RCAHMW), 2017.