A relatively small-arched aqueduct carrying the Llangollen Canal over the Afon Eglwyseg. The aqueduct comprises of one large arch over the river, together with a second (small culvert) arch to the west for the tail race of the Pente-felin corn-mill which runs down to the River Dee. It was flanked by large masonry retaining walls which extend some 35 metres on either side. The aqueduct was designed between the enabling Act for the navigable canal feeder in 1804 and was completed in 1808 when Thomas Telford was general agent of the Ellesmere Canal Company and Thomas Denson was the resident engineer (Charles Hadfield, 'Canals of the West Midlands', David & Charles 2nd ed. 1969, pages 178-79).

In contrast to the innovative technology used on the Ponycysyllte and Chirk Aqueducts (iron and hydraulic mortar and brick), the Afon Eglwyseg aqueduct is of conventional construction using puddling-clay waterproofing behind masonry. Similar traditional aqueducts were built elsewhere on the Llangollen section of the Ellesmere Canal such as at Cross Street near Froncysyllte and the three brick-arch aqueduct over the River Perry near Welsh Franckton in Shropshire, on the Llangollen line of the Ellesmere Canal's Llanymynech Branch.

The Eglwyseg structure is given an impessive height on its lower downstream side because of the elevated line of the canal. Here as the canal approches the afon Eglwyseg valley it is carried on a substantial bank, which then becomes a long stone-revetted causeway as it crosses the valley. This is characteristic of a number of early nineteenth-century railway causeways such as that located on the Oernant canal-linked railway just upstream of the aqueduct. From the plan of the aqueduct and the bi-convex form of the river arch, it can be suggested that such stone-revetted wing walls were an after thought, and originally the wing-walls were designed to be braced in the massed earth of the relatively high approach embankments.

The mass of masonry rising above the semi-circular river arch on the high downstream elevation is capped by a projecting block-masonry string-course at canal deck level. There is a particularly narrow masonry parapet formed by the unorthodox device of placing large square stone-blocks vertically.

The cut for the canal, and the north face of the aquaduct was rebuilt around 1990 (Cadw Listed Building description no. 19683).

The Eglwyseg Aqueduct and retaining walls is a picturesque example of an eighteenth-century type canal aqueduct combined with an early nineteenth-century masonry causeway associated with Thomas Telford, an internationally significant innovative engineer.

Stephen R. Hughes, RCAHMW, 20 March 2007.