CONTEXT 176 : JUNE 2023 21 WORLD HERITAGE SITES as the southerly section. The eastern section to Chirk Bank was constructed fairly rapidly. From Gledrid Bridge, as the land rises from the Shropshire Plains, it begins to cling to the side of the Ceiriog Valley, cut into the landscape and constrained by cuttings to one side and man-made embankments to the other. For a short period, the canal terminated at Chirk Bank, with wharves to service the nearby ironworks, collieries and quarries, while the next monumental section of canal to Trevor was completed. While the canal route to Chirk Bank might have been recognisable to those who were conversant with earlier 18th-century canals in the UK, what followed was a testing ground for ideas that would change engineering practice internationally, and would prove ground-breaking. Initially emerging from the hillside on a high man-made embankment, the canal transitions into a stone aqueduct with 10 stone arched piers carrying the navigation 21 metres above the River Ceiriog, higher than any previous navigable aqueduct. While the external form might appear quite traditional (similar, for instance, to the mass masonry Lune Aqueduct in Lancaster of 1797), it has a unique experimental, composite construction. A trough base is formed of cast-iron plates, bolted together, with brick sides, rather than the usual puddled clay, clad in stone. This, combined with hollow spandrels, reduced the overall weight of the structure and allowed for a more slender form. Having crossed the Ceiriog Valley, the canal engineers were immediately faced with a ridge of high ground. The decision was made to excavate a tunnel rather than respond to the topography. Although canal tunnels were already widespread across the inland waterway network, the 471-metre-long Chirk Tunnel was possibly the first to have an integral towing path for horses, rather than requiring manpower to ‘leg’ the boat through by lying on the roof of the boat and walking along the roof or walls of the tunnel. Having traversed a number of deep cuttings and embankments, the canal passes through a shorter tunnel of similar construction at Whitehouses, a little further to the north. The centrepiece of the world heritage site is the internationally renowned Pontcysyllte Aqueduct. Given its late-18th-century context, the structure demonstrates a courageous and creative application of new ideas, combined with an expert knowledge in the handling of new materials that would come to exemplify the work of Jessop and Telford. Spanning 307 metres over the River Dee valley on 18 enormous tapered stone piers, and supporting a cast-iron trough at a maximum height of 38 metres above the river, its proportions alone are awe inspiring and unlike anything else seen before, but the confident and elegant early application of cast iron at this scale is what makes it truly extraordinary. Building on both Telford’s pioneering work in cast iron at his rudimentary iron aqueduct at Longdon-on-Tern of 1796 and Jessop’s initial use of the material alongside his partner Benjamin Outram in a cast-iron aqueduct on the Derby Canal during a similar period, the pair were confident enough in the capability and application of the material to suggest an at-grade aqueduct to span the Dee Valley, which could not have been accomplished with any other material. Heading west from Trevor Basin, the canal is noticeably narrower, and clings, sometimes precariously, to the northern side of the Dee valley, once again constrained by cuttings and man-made embankments. Perhaps the most impressive of these is at Wern-Isaf on the eastern approach to Llangollen, where the northern bank is cut deep into the rock, producing a sheer cliff face, the spoil from which was deposited on the southern down slope to create the embankment. This has now become wooded, creating a unique, pastoral atmosphere. Beyond Llangollen the navigable canal terminates at Chain Bridge, while the narrow feeder channel continues further up the valley The Chirk Tunnel may have been the first to have an integral towing path for horses. The Longdon-onTern Aqueduct was built in 1796 to carry the Shrewsbury Canal across the River Tern.
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