Crawia slate mills were located about half a kilometre to the north of Llanrug. Two sites were in use, on either side of the Caernarvon and Llanberis branch line of the L&NWR, along with a third site nearby to the south-east at Pontrhythallt (NPRN 420118). The three sites were located respectively at SHSH53646426 (a), SH53576421 (b) and SH5406406 (c). They operated from the 1880s to the 1920s processing slate into writing slates and slab products. They used material imported from Dinorwig brought to the eastern end of the site along the Padarn Railway. Finished product left on the L&NWR branch to Caernarfon. The mills (a & c) were water powered in tandem: the shallow fall of the ground forced the works to be widely separated. Mill (b), adjacent to the river, had its own leat. By 1916 mill (b) was annotated on the local map as 'New Crawia Slate Works', while that at (a) was portrayed as 'Crawia Mills (School Slates)'.
In 1991, vesiges of buildings, one with a waterwheel axle, could be seen at the eastern end of the site. At the western end some buildings were in re-use including a house and mill converted into a dwelling. Most of 'the leat' was traceable (presumably the one supplying (a) and (c)). Also traceable was a tramroad formation which connected (a) and (c) with each other and with the railway siding.

Sources:
D.Gwyn, Welsh Slate: the Archaeology and History of an Industry (RCAHMW 2015), p.238.
Ordnance Survey County Series 25-inch maps: sheets Caernarvon XI.14 & XI.15.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p.34.

David Leighton, RCAHMW, 15 June 2015