The Tern Valley Trail
By the late 18th Century, Shrewsbury was largely dependant on the many mines centred around the Oakengates area for its coal supplies. In fact, the sheer volume of traffic used for transporting raw materials to the county town had rendered the Holyhead Road all but impassable in places and an alternative was sought in 1792 when a potential route for a new canal was first surveyed. In 1793, an Act of Parliament sanctioned a 17-mile canal linking Shrewsbury to the existing waterways of the east Shropshire coalfield at Trench. The civil engineer Josiah Clowes was charged with overseeing the project but, after his death in 1795, Thomas Telford was appointed his successor. The Shrewsbury Canal, which opened in several stages between 1794 and 1796, included many groundbreaking engineering feats along its length, of which perhaps the most important is situated at Longdon-on-Tern.
The Oldest Surviving Iron Aqueduct in the World
When Thomas Telford became chief engineer of the Shrewsbury Canal project, one of his first tasks was to bridge the River Tern at Longdon. A masonry aqueduct begun by his predecessor had been swept away in the floods of February 1795 and Telford was persuaded by the canal company’s promoters to consider a new material for its replacement – iron.
Longdon Aqueduct is the oldest surviving cast iron structure of its kind in the world but was not the first to be built. A few weeks before its completion, in February 1796, the much smaller Holmes Aqueduct, designed by the civil engineer Benjamin Outram, opened on the Derby Canal. Sadly, this historic structure was demolished in 1971.
All the way from Ketley
The idea of utilising a cast iron aqueduct is credited to Thomas Eyton, Chairman of the Shrewsbury Canal Company, although Telford would certainly have been aware of developments at Derby through his acquaintance with Outram’s business partner William Jessop. The two men had previously worked together on the Ellesmere Canal, where they had apparently already considered using iron. Another of the project’s promoters, the Ketley ironmaster William Reynolds, co-designed the aqueduct with Telford and supplied the iron plates for the frame of the structure.
Using the surviving ends of Josiah Clowes’ water ravaged edifice, a trough and separate towing path measuring 62 yards in length were slotted into place on knee-braced supports just 16 feet above the Tern Valley. Although Longdon Aqueduct is now a scheduled ancient monument, its principal value lies in the influence it had on Telford’s later work. The experience he gained from using iron at Longdon was to prove invaluable in the construction of the Pontcysylite Aqueduct on the Llangollen Canal, one of the great engineering triumphs of the canal age.