Beacons Reservoir Dam including draw-off tower, bridge and spillway

Beacons Reservoir Dam, built in the late 19th century, features a clay core and bunded earth construction. The inner face of the embankment is finished with pitched stonework, which ends in a low wave-wall that retains the roadway. The outer face is turfed.

To the east of the dam stands the draw-off tower, which is set on a masonry platform that extends from the dam's inner face. This circular tower is constructed from coursed, rock-faced masonry and has a corbelled embattled parapet and a battered base. It includes small windows with tooled stone chamfered surrounds on the ground floor facing north-east and north-west, and small loop openings on the first floor facing south-east and south-west. The front features machicolation beneath the parapet, flanking stepped buttresses, and a doorway with a segmentally arched head that integrates into the reveals of a taller arched recess. A framed plaque above commemorates the reservoir's opening in 1897. The base platform is made of rock-faced stonework and includes iron rails.

At the eastern end of the dam is the spillway, which consists of a long stepped cascade that curves down from a stepped weir, connected at the bottom by a channel from a tunnel outlet. The weir is supported by stone-faced circular piers and features granite steps. A small stone bridge crosses the spillway, providing access to the top of the dam, and showcases finely-worked masonry revetment walls and a pitched stone channel surface. At the base of the spillway, the western revetment wall curves around to a parabolic-arched tunnel entrance with a masonry portal that includes a cornice and blocking course. The revetment walls extend to the bridge that carries the Hirwaun Road. The masonry is of high quality, featuring rock-faced squared stone with tooled leading edges, a raised plinth, and large rock-faced coping stones along the parapet. The bridge itself has a shallow segmental arch with voussoirs and a keystone, and the parapets end in square piers topped with massive rock-faced stone caps. Stone walls extend north and south from the east end of the bridge.