A clue is know available that should help you!
Chirk Aqueduct is a 70-foot (21 m) high and 710-foot (220 m) long aqueduct that carries the Llangollen Canal across the Ceiriog Valley near Chirk, in Wales.
The aqueduct was designed by Thomas Telford and completed in 1801. It possesses a cast iron trough within which the water is contained. The masonry walls effectively hide the cast iron interior. The aqueduct followed Telford's innovative Longdon-on-Tern Aqueduct on the Shrewsbury Canal, and was a forerunner of the Pontcysyllte Aqueduct also on the Llangollen Canal.
Telford pioneered the use of cast iron in bridges as well as aqueducts, and cast iron troughs were widely used elsewhere on the British canal network, especially where a secure and watertight crossing or bridge was needed. Another famous example is the Cosgrove Aqueduct on the Grand Junction Canal at Milton Keynes in Buckinghamshire.
The aqueduct consists of ten arches, each with a span of 40 feet (12 m). The water level is 65 feet (20 m) above the ground and 70 feet (21 m) above the River Ceiriog. The first stone was laid on 17 June 1796. William Hazledine provided the ironwork for the aqueduct.Side plates were added to the aqueduct in 1870.
From Wikipedia
While your in the area why not check out some other caches.
Tunnel Vision by Team Marizpan
SideTracked : Chirk by Team Marizpan
The Zodiac Series - Aquarius by Team Maddie UK
The Zodiac Series - Libra by Team Maddie UK