Late on the damp and gloomy afternoon at the end of February I made a short industrial archaeology pilgrimage to one of the most breath taking (literally) industrial monuments in Britain – the Pontcysyllte Aqueduct. This engineering marvel still carries the Llangollen Canal across the River Dee via a cast-iron trough. Completed in 1805 it is the longest aqueduct in Britain and the tallest canal aqueduct in the world. This latter fact I could fully appreciate standing in the middle of the aqueduct on the seemingly narrow public footpath, with just a steel rail separating me from a drop of 38m to the rushing waters of the Dee.
The engineering dimensions are impressive, the aqueduct being 307m (336 yd) long, 3.7m (12 ft) wide and 1.6m (5 ft 3 in) deep. The water channel is formed by a cast iron trough supported by arched iron ribs carried on 18 hollow masonry pillars, each 16m (53 ft) wide.
The aqueduct was designed by the transport engineer Thomas Telford, who also engineered the nearby A5 toll road to Holyhead and is regarded as one of his greatest civil engineering achievements.
It was the central section of the unfinished Ellesmere Canal, begun in the 1790s. The intention was to build a route linking the River Severn at Shrewsbury with the Mersey estuary and the port of Liverpool to the north via the coalfield fields around Wrexham. William Jessop’s original plan was to create two flights of locks on either side of the Dee valley with an embankment and small stone aqueduct carrying the Ellesmere Canal across the river. After Telford was hired that plan was changed to a single aqueduct spanning the steeply-sided river valley. The work, which began in July 1795 took ten years to construct, cost around of £47,000, and opened to traffic on 26 November 1805. The cost of building this stretch of the canal was probably a significant factor in the failure to complete the project. It was not until the mid-1830s that the aqueduct was finally linked to the rest the Midlands canal network.
Standing beneath the ribbed iron arches of the aqueduct, with the mist drifting around the piers, I could appreciate the engineering design that led to this monument being inscribed on UNESCO’s World Heritage Site list. That inscription cites the following reasons for its importance:
Criterion (i): The Pontcysyllte Aqueduct is a highly innovative monumental civil engineering structure, made using metal arches supported by high, slender masonry piers. It is the first great masterpiece of the civil engineer Thomas Telford and formed the basis of his outstanding international reputation. It bears witness to the production capacities of the British ironmaking industry, which were unique at that time.
Criterion (ii): The intensive construction of canals in Great Britain, from the second half of the 18th century onwards, and that of the Pontcysyllte Canal in particular in a difficult region, bear witness to considerable technical interchanges and decisive progress in the design and construction of artificial waterways.
Criterion (iv): The Pontcysyllte Canal and its civil engineering structures bear witness to a crucial stage in the development of heavy cargo transport in order to further the Industrial Revolution. They are outstanding representatives of its new technical and monumental possibilities.
The only better way to appreciate the vision and bravery needed to conceive an iron trough, supported by 18 stone piers 38m high, as a way to span the Dee valley is to take a canal boat across it. However, I’m not sure I’m brave enough to do that, so I’ll content myself with the views from the towpath and the safety provided by gripping the metal rail whilst taking in the Welsh valley scenery.