One of three slate mill sites in Cwmorthin quarry. The lower Mills were probably established in the late 1860s or early 1870s, as two parallel slate processing buildings (the Old Lower and Southern Mills) side by side. The lower mill was later extended to the south and another mill, London Hall, erected at its north end, the latter on land reclaimed by arching over the river. Power was supplied by a 30HP waterwheel in a pit below the floor of the Old Lower Mill. In 1900 there were 21 saw tables and 21 dressing machines.
In 1925, after a quarter-century lying idle, the Lower Mill and London Hall were repaired. There were eight saw tables and dressing machines in London Hall and four of each in the Old Lower Mill, and the waterwheel was replaced by a 25HP low-head Pelton wheel.
In 2008 only parts of the footings of these buildings remain. The wheel pit is roofed over, with a new cover on the turbine, and a saw table and short length of tramway have been laid in a landscaped setting, the whole now derelict again.
W J Crompton, RCAHMW, 5 November 2008.