The impressive three-storeyed Ynys-y-pandy slate processing works, which served the Gorseddau Quarry, was built in 1856-7 by Evan Jones of Garndolbenmaen and probably designed by James Brunlees. It is ingeniously planned so that the natural fall of the site assisted the manufacturing process. A deep trench inside accommodated a large overshot water wheel (26 ft, 8m in diameter), and on the south side a long curving ramp brought branches of the tramway from Gorseddau Quarry into the mill at two different levels, serving the middle and upper floors. The grand, round-headed openings are closely spaced like a Roman aqueduct. The eastern gable is surmounted by a decorative feature incorporating a false shimney stack, and the west gable windows have at some time had window frames or shutters. Otherwise the construction is bold and plain but none the less impressive.

The mill specialised in the production of slate slabs for floors, dairies, troughs, urinals, etc. In its heyday, in 1860, it was producing over 2,000 tons per annum, but seven years later production was down to 25 tons per annum (due to poor quality of the quarried slate) and the business went into liquidation in 1871. The building provided a venue for eisteddfodau until the roof was removed around 1906.