During construction of the Shrewsbury Canal the difficulty of crossing the river Tern at Longdon had to be overcome. It was decided to construct a traditional masonry aqueduct but before completion heavy flooding washed it away. Thomas Telford designed a replacement for it but this time it was to be constructed of cast iron. The trough was to be formed from cast-iron plates fastened together with wrought iron nuts and bolts. The plates were flanged to increase their strength and facilitate fastening them together. The side plates were wedge-shaped, in the manner of voussoirs in a masonry arch. The trough was designed to be narrow, so boats were unable to pass, and the towpath was cantilevered outwards. The completed aqueduct was 62 feet long and it was laid along the top of three fabricated cast-iron supports and on brick and masonry abutments at each end. The latter were most likely the remains of the earlier aqueduct that was washed away. The trough consisted of four spans, one inner span over the river, and the outer spans springing from the abutments. The aqueduct remained in water until the 1950s and it still stands as a lasting monument to Thomas Telford.
It is now accepted that Telford used this aqueduct as the prototype for the massive Pontcysllte Aqueduct, which was needed to carry the Llangollen Canal (formerly the Ellesmere Canal) over the wide valley of the river Dee at Ruabon. The Longdon-on-Tern Aqueduct opened one month after Benjamin Outram's small aqueduct at Derby was opened in March 1796. Telford must have noted that considerable effort was required to haul a boat along the trough of the Longdon-on-Tern Aqueduct, as it was difficult for displaced water to pass between the sides and bottom of the boat and the trough itself. As a result, on the Pontcysllte Aqueduct the trough was made wider and the towpath was cantilevered inwards over the water. However, the trough was insufficiently wide for boats to pass.
In order to cross the Dee valley, Telford constructed 18 stone piers, firmly embedded in sandstone rock, and the fabricated trough was laid along the top of these. Like the Tern Aqueduct, the side plates were wedge-shaped, in the manner of voussoirs in a masonry arch. The cast-iron plates were manufactured at the Ketley ironworks of William Reynolds. The aqueduct is more than 1,000 feet long, the trough is 11 feet 10 inches wide and it stands around 120 feet above the river Dee. It took 10 years to construct and it opened in 1805.