Still continuing on the canal theme, I visited the PonycysyllteAqueduct on the Llangollan canal in north Wales. The adqueduct bridges the River Dee valley and was built by Thomas Telford and William Jessop in 1795 and construction was completed in 1800. The structure is 307m long, 3.4 m wide and 1.60 m deep. It consists of a cast iron trough supported 38 m above the river on iron arched ribs carried on nineteen hollow masonry piers (pillars). Each span is 16 m wide. The trough is drained annually to allow periodic maintenance to be carried out.