The ruins of this very large slate mill complex stand on made-up ground on the col between Cwmorthin and Cwm Croesor. The buildings have a complex history. The first part, of large sawn blocks with semicircular arched openings one of which still stands, may have been planned in conjunction with the driving of No 4 adit in 1857-59 (NPRN 408860), and was in existence by 1862. In 1869-70 a large extension was built to the east, of smaller sawn blocks, and a few years later a long narrow extension was added to the west. In 1909-10 the first and second phases were completely rebuilt, with new roofs and machinery and with extensive stacking areas to the north. From this date the reconstructed building housed 20 saw tables and 20 dressing machines, whilst the west extension had 10 of each. Power was derived from a waterwheel in a pit between the two buildings, where a vertical shaft still survives; in its latest form the wheel could have been up to 12.2 metres in diameter and 1.5 metres wide.
A lean-to at the west end of the west extension existed prior to 1887, and was a fitting shop. To the south of the west extension a row of four buildings was complete by 1867, and may have been barracks before becoming stores. A small smithy was built prior to 1867 south of the 1869-70 extension, but was replaced in the 1890s by a new smithy on the east side of the Level 9 adit haulage wheelpit (see NPRN 40601).
The mill was silent from 1914 to 1919, and again from 1930 to 1947. Final closure came in 1948, after which the roof was stripped and walls were pushed over to extract blocks for splitting. Part of the south wall of the 1909 mill still lies on the ground in one piece.
W J Crompton, RCAHMW, 6 May 2009.