Arches of the Pontcysyllte Aqueduct. At 307 metres, this aqueduct is the longest and highest cast-iron aqueduct in the world. It carries the Llangollen canal 38 metres above the River Dee. It was designed by Thomas Telford and constructed between 1795 and 1805. The iron castings were produced at the Plaskynaston foundry and each casting dovetails into the next. The joints were made watertight using welsh flannel dipped in boiling sugar, after which the joints were sealed with lead. The aqueduct is a grade 1 listed building. It is still used for its original purpose and is crossed by more than 1000 boats each year.