Last edited 23 Oct 2023

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Pontcysyllte Aqueduct and Canal World Heritage Site

The monumental scale of the engineering and the innovative use of materials, most notably cast and wrought iron, make this world heritage site universally outstanding.

Chirk Aqueduct.jpg
Chirk Aqueduct was completed in 1801 by William Jessop and Thomas Telford.

Pontcysyllte Aqueduct and Canal World Heritage Site, inscribed by Unesco in 2009 and predominantly owned by the Canal and River Trust, comprises the westernmost 18-kilometre length of the Llangollen Canal, along with its associated engineering and architectural features. It extends from Gledrid Bridge, near Rhoswiel, via Trevor Basin to its western terminus at Horseshoe Falls near Llantisilio.

This length of the Llangollen Canal was constructed between 1795 and 1808 to the designs of William Jessop and Thomas Telford, two of the most notable figures of the ‘heroic age’ of civil engineering during the industrial revolution. This world heritage site is unusual in that it straddles two nations and three local authority areas. Located primarily in north-east Wales, it runs through the Welsh county of Denbighshire and the county borough of Wrexham, before crossing the border into the English county of Shropshire near the small town of Chirk.

Originally the canal from Gledrid Bridge to Trevor was known as the Ellesmere Canal, and the section west to Horseshoe Falls was its Llangollen branch, but in the late 20th century both lengths were rebranded the Llangollen Canal. The Ellesmere Canal was constructed under an Act of Parliament passed in 1793, with construction commencing in 1795. Originally the canal was planned as a means of linking the major river arteries of the Mersey, Dee and Severn, and opening them up to the rich slate, coal and iron reserves of the Welsh uplands. However, only part of the original plan was eventually executed, with the stretch from Chester south to Ruabon abandoned, along with the link south to the River Severn. This meant that that the canal now terminated at Trevor Basin, where wharves and a tramway served the local iron and coal industries.

Without a connection to the intended supply of water near Wrexham, a new supply was required to keep the Ellesmere Canal in water. The 10-kilometre Llangollen Branch was constructed under a separate Act of 1804, primarily as a feeder and to supply water to local industries; for this reason it is not as wide as the southerly section. The eastern section to Chirk Bank was constructed fairly rapidly. From Gledrid Bridge, as the land rises from the Shropshire Plains, it begins to cling to the side of the Ceiriog Valley, cut into the landscape and constrained by cuttings to one side and man-made embankments to the other. For a short period, the canal terminated at Chirk Bank, with wharves to service the nearby ironworks, collieries and quarries, while the next monumental section of canal to Trevor was completed. While the canal route to Chirk Bank might have been recognisable to those who were conversant with earlier 18th-century canals in the UK, what followed was a testing ground for ideas that would change engineering practice internationally, and would prove ground-breaking.

Initially emerging from the hillside on a high man-made embankment, the canal transitions into a stone aqueduct with 10 stone arched piers carrying the navigation 21 metres above the River Ceiriog, higher than any previous navigable aqueduct. While the external form might appear quite traditional (similar, for instance, to the mass masonry Lune Aqueduct in Lancaster of 1797), it has a unique experimental, composite construction. A trough base is formed of cast-iron plates, bolted together, with brick sides, rather than the usual puddled clay, clad in stone. This, combined with hollow spandrels, reduced the overall weight of the structure and allowed for a more slender form. Having crossed the Ceiriog Valley, the canal engineers were immediately faced with a ridge of high ground. The decision was made to excavate a tunnel rather than respond to the topography. Although canal tunnels were already widespread across the inland waterway network, the 471-metre-long Chirk Tunnel was possibly the first to have an integral towing path for horses, rather than requiring manpower to ‘leg’ the boat through by lying on the roof of the boat and walking along the roof or walls of the tunnel. Having traversed a number of deep cuttings and embankments, the canal passes through a shorter tunnel of similar construction at Whitehouses, a little further to the north.

The centrepiece of the world heritage site is the internationally renowned Pontcysyllte Aqueduct. Given its late-18th-century context, the structure demonstrates a courageous and creative application of new ideas, combined with an expert knowledge in the handling of new materials that would come to exemplify the work of Jessop and Telford. Spanning 307 metres over the River Dee valley on 18 enormous tapered stone piers, and supporting a cast-iron trough at a maximum height of 38 metres above the river, its proportions alone are awe inspiring and unlike anything else seen before, but the confident and elegant early application of cast iron at this scale is what makes it truly extraordinary. Building on both Telford’s pioneering work in cast iron at his rudimentary iron aqueduct at Longdon-on-Tern of 1796 and Jessop’s initial use of the material alongside his partner Benjamin Outram in a cast-iron aqueduct on the Derby Canal during a similar period, the pair were confident enough in the capability and application of the material to suggest an at-grade aqueduct to span the Dee Valley, which could not have been accomplished with any other material.

Heading west from Trevor Basin, the canal is noticeably narrower, and clings, sometimes precariously, to the northern side of the Dee valley, once again constrained by cuttings and man-made embankments. Perhaps the most impressive of these is at Wern-Isaf on the eastern approach to Llangollen, where the northern bank is cut deep into the rock, producing a sheer cliff face, the spoil from which was deposited on the southern down slope to create the embankment. This has now become wooded, creating a unique, pastoral atmosphere.

Beyond Llangollen the navigable canal terminates at Chain Bridge, while the narrow feeder channel continues further up the valley to terminate at Horseshoe Falls. This unique and distinctively curved cast-iron and masonry weir, 140 metres long, impounds water from the River Dee to supply the Ellesmere Canal and to help supply drinking water to southern Cheshire.

Somewhat overlooked among the other, more elaborate, structures within the world heritage site is a further cast-iron innovation found within three relatively modest stone bridges that traverse the canal. These unusual composite bridges employ curved cast-iron beams supporting shallow masonry arches. These allowed the necessary headroom beneath the span while reducing the required gradient of the approach road to either side.

Pontcysyllte Aqueduct and Canal is by no means the only canal-based world heritage site in the world. Canals feature heavily in some world heritage site cities such as Strasbourg, Venice, Bruges and Amsterdam, and there are others inscribed specifically because of the historic importance of man-made water systems. The Grand Canal in China has its origin as far back as the fifth century BC, its extensive network of communication waterways being recognised as the earliest man-made waterway network in the world. The 360-kilometrelong Canal du Midi in south-western France also pre-dates the Llangollen Canal by a century and is recognised as having influenced the pioneers of British canal construction. The 202-kilometre-long Rideau Canal in Canada, completed in 1832, was a strategic military canal. It utilised technology already tried and tested in the UK and Europe, such as on the Ellesmere Canal, assisting the British to defend the colony of Canada against the USA.

However, it is the monumental scale of the engineering, and the innovative use of materials, most notably cast and wrought iron, that make Pontcysyllte Aqueduct and Canal World Heritage Site universally outstanding. At the time of its completion, the length of canal was described as being, ‘composed of works more difficult of execution than can perhaps be found anywhere within an equal distance of canal navigation’, in part due to the desire of its engineers to overcome the difficult terrain of two major river valleys, and the ridge of ground between them, without wishing to follow the topography or create time-consuming and resource-hungry lock flights.

The inscribed length of the Llangollen Canal is multi-designated, and is both a Cadw and Historic England scheduled monument. It also bisects the Offa’s Dyke scheduled monument west of Pentre. In addition, there are 25 listed structures (bridges, tunnels, aqueducts, culverts and weirs) directly associated with the navigable canal channel itself, and a number of buildings and structures relating to the historic operation of the canal. The principal structure within the world heritage site, Pontcysyllte Aqueduct, is Grade I listed, while Chirk Aqueduct and Chirk Tunnel are both listed at Grade II*.

There are three conservation areas along the length of the inscribed section of canal, two relating to the settlements of Chirk and Llangollen, the other relating to the area around the settlement of Froncysyllte, the Pontcysyllte Aqueduct and Trevor Basin. As well as these heritage designations, the world heritage site runs through the Clwydian Range and Dee Valley Area of Outstanding Natural Beauty. All these designations can increase the level of complexity around the management of the canal as a working recreational asset.

Pontcysyllte Aqueduct and Canal World Heritage Site is managed by a stakeholder partnership, overseen by the World Heritage Site Strategic Board with three delivery subgroups for visitor management and economic regeneration; planning, landscape and conservation; and learning. The partnership comprises members of the Canal and River Trust as primary landowner, the three local authorities, and statutory advisers from Cadw, Historic England, ICOMOS and the area of outstanding natural beauty. However, with its designation boundary closely following the canal corridor, the principal day-to-day operation of the canal and the maintenance of many of its associated structures is the responsibility of the Canal and River Trust.

The Canal and River Trust is a charity created in 2012 to safeguard and maintain over 2,000 miles of inland waterways across England and Wales. Although primarily a health-and-wellbeing charity, recognising and promoting the positive value that blue and green spaces can have on the physical and mental health of the population, the trust has heritage conservation very much at its heart, owning and caring for the third largest historic estate in the UK, after the Church of England and the National Trust.

At the time of writing, the trust owns 2,707 listed buildings and 47 scheduled monuments, and has canals passing through seven registered parks and gardens and six historic battlefields. The trust also has canals running through four designated world heritage sites in England and Wales (Bath, Blaenavon, Saltaire and Pontcysyllte), with only Pontcysyllte designated specifically for its canal-related infrastructure.

The inscribed length of canal is part of a 19-kilometre ‘pound’ that has no locks, so the consequences of failure of any of the assets would be potentially catastrophic. There are records of several breaches within the inscribed area at Bryn Howel, Sunn Trevor and Chirk Bank. These are all attributable to failures of the man-made embankments on the downhill side of the canal, resulting in large areas having been relined with concrete over the decades.

As well as having a detailed heritage policy and standards to ensure that our heritage assets are maintained and repaired in an appropriate way, the Canal and River Trust has a robust national asset management policy that ensures we keep high-quality data and information to assist us in making informed decisions about maintenance and repair. The asset inspection procedure defines the requirements and responsibilities for the inspection of operational infrastructure and assets. A hierarchical and cyclical three-stage inspection process comprises bi-monthly visual safety and service inspections, biennial general inspections, and principal inspections, the cycle of which will be between six and 24 years, depending on the condition and risk of the asset, determined by a risk matrix. All of these inspections are undertaken by appropriately certified inspectors within the Canal and River Trust.

With nearly 350,000 visitors each year, primarily concentrated around the Pontcysyllte Aqueduct and Trevor Basin, balancing the requirements of modern health and safety and the conservation of a 230-year-old asset with the expectations and needs of visitors can be a challenge. The Canal and River Trust is working with the partnership, and other consultants, to look at how visitors can be better managed, and whether physical interventions are required to improve visitor safety while seeking to minimise any potential impacts on the significance and outstanding values of the site. A site-wide masterplan has been drawn up in collaboration with the partnership, and the landscape, planning and conservation sub-group. The first elements of this are now being implemented to better open up the canal basin, improve visitor access and provide better facilities.

This article originally appeared in the Institute of Historic Building Conservation’s (IHBC’s) Context 176, published in June 2023. It was written by Peter Chowns, principal architect and conservation specialist at the Canal and River Trust.

--Institute of Historic Building Conservation

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