Cross Mill is one of three slate mills developed by Cwmorthin Quarry (NPRN 40594), probably in the mid-1860s. It lies at the foot of the main surface incline (NPRN 408415) connecting the Mill level with Levels 2, 3, 4, 6, 7 and 8. Built of sawn slate blocks and with triangular-headed windows, it contained two rooms 19 x 10 metres, divided by a waterwheel chamber. Water came from an intake just below the outlet of Lake Cwmorthin, via a leat and wooden launder on the west side of the river, crossing at high level to the wheel; some of the rubble stone pillars which supported the launder are still in situ. In 1900 one room had five saw tables and dressing machines, the other housed timber sawing machinery. In 1925 it was not thought economic to repair the mill and it was used as a smithy and carpenters' shop.
By 2008 the Cross Mill had been totally demolished, and not even its footings survive.
W J Crompton, RCAHMW, 5 November 2008.