A cast-iron legacy

The Canal & River Trust care for over 300 aqueducts across the waterways of England and Wales. For the most part, they are constructed from traditional materials, such as brick or masonry. There are, however, a number of exceptions, impressive structures wrought from cast iron. Follow our heritage advisor, Bill Froggatt, as he takes a closer look at the history of some of these stunning feats of engineering.

A narrowboat crosses the Pontcysyllte Aqueduct The Pontcysyllte Aqueduct on the Llangollen Canal

Coal and iron were the raw materials that powered the Industrial Revolution, defining an age and changing the way we live forever. Iron, in particular, has left an indelible mark on our landscape, in the shape of bridges, buildings and railways.

It all began in the latter half of the 18th century, when British engineers pioneered the use of cast iron, creating imposing structures such as the Iron Bridge on the River Severn, the world’s first cast-iron bridge, opened in 1779. It wasn’t long before canal engineers spotted the potential of this new ‘miracle’ metal, and by the 1790s, cast-iron aqueducts began to spring up across the country.

Perhaps the most celebrated of these is the Pontcysyllte Aqueduct on the Llangollen Canal. At over 300 metres long and carrying the canal 38 metres above the River Dee, it’s one of the longest and highest aqueducts in the country. Opened in 1805, the design of the aqueduct is largely attributed to Scottish civil engineer, Thomas Telford, though at least some of the credit should go to the canal’s principal engineer, William Jessop, who oversaw much of the construction. Its slim arches, a key feature of cast-iron aqueducts, enable the canal to be carried over the valley with little impediment to the river flowing below, thus reducing the risk of flood damage.

Today, the Pontcysyllte Aqueduct is heralded as a wonder of engineering, sitting at the heart of a UNESCO World Heritage Site on the Llangollen Canal, attracting thousands of eager visitors every year.

As impressive as it is, the Pontcysyllte Aqueduct was by no means the first of its kind. That particular distinction belongs to the Holmes Aqueduct, brainchild of civil engineer, Benjamin Outram. Built to carry the Derby Canal over a stream, the Holmes Aqueduct was the world’s first navigable cast-iron aqueduct, opened in 1796, nearly 10 years before Pontcysyllte. Significantly shorter than the Pontcysyllte at only 14 metres long, it didn’t have the same artistic resonance as its counterpart, and sadly, after several repairs, it was finally scrapped in 1970 to make way for a ring road.

Stalybridge Aqueduct on the Huddersfield Narrow Canal Stalybridge Aqueduct on the Huddersfield Narrow Canal

Another of Outram’s creations, Stalybridge Aqueduct, has stood the test of time. Built in 1800 to replace a masonry aqueduct damaged by flooding, it is the oldest navigable cast-iron aqueduct still in use and carries the Huddersfield Narrow Canal over the River Tame.

For a variety of reasons, as successful and durable as these structures proved to be, they never really caught on. In part due to the inherent conservatism of canal companies and a lack of skilled ironworkers, by the turn of the century, the construction of cast-iron aqueducts had stalled.

There were some notable exceptions, like the Stanley Ferry Aqueduct on the Aire & Calder Navigation, built by George Leather in the 1830s and reputedly the largest cast-iron aqueduct in the world. But for the most part, the age of the cast-iron aqueduct was over before it even began, as canal companies quickly reverted to the tried and tested construction methods of brick and masonry.

Stanley Ferry Aqueduct inspired both the Tyne and Sydney Harbour Bridges Stanley Ferry Aqueduct inspired both the Tyne and Sydney Harbour Bridges

While cast-iron aqueducts may have been the exception rather than the rule, their influence has nonetheless echoed down the years. We can plot a direct line from structures like the Stanley Ferry Aqueduct, with its distinctive arches, right through to extraordinary feats of 20th century engineering, such as the Tyne Bridge in Newcastle and the world-famous Sydney Harbour Bridge in Australia.

Without pioneering visionaries like Outram, Leather and Telford, we wouldn’t have the infrastructure we have today. Although their names are not as familiar as the likes of Wren and Kingdom Brunel, they have left their own mark on history, a legacy built from iron plates, bolts and rivets. Hopefully, the impressive cast-iron structures that they created will stand forever as monuments to the bravery, ambition and ingenuity of these remarkable men.

Last date edited: 11 March 2022

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