The working of slate at Conglog began in 1854 with a two-year take-note to Robert Roberts, Surgeon and John Williams, quarryman, of Ffestiniog. It continued under a number of leases and sub-lettings, and under several different trading names, until 1909. The new Conglog Slate and Slab Co Ltd went into liquidation in 1891, and a new leaseholder from 1895 traded as the Glyn Ffestiniog Quarry.
The complex includes four open pits leading to underground levels and chambers in the Back Vein and North Vein of the Blaenau Ffestiniog sequence (NPRN 408348), a weighing house (NPRN 408349), gravity incline (NPRN 408356) leading down to a water-powered mill (NPRN 408357), and areas of tipped waste (NPRN 408358 and 408359). Tan-yr-allt, an adjacent terrace of cottages, a former stable, and the nearby detached house Plas Cwmorthin were built for the use of Rhosydd Quarry (NPRN 40601).
W J Crompton, RCAHMW, 16 October 2008.