The Dam at Lake Vyrnwy was started in 1881 and completed seven years later in 1888. It was the first large stone-built dam in the United Kingdom, and is built partly out of great blocks of Welsh slate. When built it cost £620,000, which today is around £22,000,000. The dam is 44 m (144 ft) high from the bottom of the valley, and 39 m (127 ft) thick at the base. The dam's length is 357 m (1172 ft), and has a road bridge running along the top. It is decorated with over 25 arches and two small towers (each with four corner turrets) that rise 4 m/13 ft above the road surface.