The Offa’s Dyke Path crosses the river Vyrnwy on a five-arch stone aqueduct, c.2km south west of Llanymynech. This was the biggest single item of infrastructure on the Montgomery Canal.
The aqueduct collapsed while it was being built from 1794 to 1796. It cost £4,500 to construct, which was more than 6% of the cost of the entire canal.
The trough for the canal water along the top of the aqueduct was lined with clay to prevent leakage. You may be able to glimpse iron bars on the sides of the structure, from various repairs and reinforcements.
To cross the valley floor and river, the canal descended from Llanymynech via a pair of locks at Carreghofa, a short walk away from here along the old towpath.
The river Vyrnwy (afon Efyrnwy in Welsh) flows east to west under the aqueduct, on its way from Lake Vyrnwy reservoir to the river Severn. The origin of Efyrnwy is unknown but probably means a river associated with a person called Ebur, or similar. It was written in 1185 as Evernoy, in 1200 as Ewernoe, and Y vernwy in 1201.
With thanks to Professor Hywel Wyn Owen of the Welsh Place-Name Society