Why this 19th-century Welsh wonder is held together by ox blood

Countries & Cities · Published 29 September 2026 Swipe more like this →

In short: Completed in 1805, the Pontcysyllte Aqueduct remains the highest canal bridge in the world. This industrial marvel, which carries narrowboats 127 feet above the River Dee, relies on a unique mortar mixture of lime, water, and ox blood to survive the harsh freeze-thaw cycles of the Welsh valley.

Designed by civil engineers Thomas Telford and William Jessop, the Pontcysyllte Aqueduct was intended to be the centerpiece of the Ellesmere Canal, a commercial artery linking the Rivers Severn, Dee, and Mersey. Construction began in 1795 and concluded in 1805, costing approximately £47,000—a significant investment at the time. The structure consists of a cast-iron trough supported by 18 hollow masonry piers, with 19 arched spans. To ensure durability, the mortar was fortified with ox blood, a traditional additive used to strengthen masonry against temperature fluctuations.

The engineering is as clever as it is bold. Telford utilized the strength of cast iron in compression, creating a trough that remains watertight despite being unfastened to the supporting arches. Because the vertical load of a boat and its cargo displaces an equal mass of water, the stress on the structure remains virtually constant regardless of traffic. The towpath is cantilevered, allowing water to flow beneath it, which helps narrowboats pass through the trough with relative ease.

Though the canal project was never fully completed as originally envisioned, the aqueduct survived due to its role as a vital water feeder for the Shropshire Union Canal. It narrowly escaped destruction in the 20th century, including a period of closure following a 1945 embankment breach that caused a train derailment. Today, the site is a UNESCO World Heritage Site, recognized for its industrial significance and its enduring role in the landscape of northeast Wales.

The name itself is often misunderstood. While some modern interpretations suggest it means 'bridge of the junction,' it actually refers to the township of Cysyllte. The structure remains a testament to Telford’s early mastery of industrial engineering, standing as a landmark that continues to require periodic maintenance, including a full drainage and inspection cycle every five years.

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