Pant Mawr was the first of the two quarries to be established, around 1850. It was situated on the western slopes of Moelwyn Mawr, and was working the same slate vein as the Moelwyn quarry, which was situated on the south eastern slopes of the same mountain. Slate quarries in the Ffestiniog region, where the slate veins slope downwards at an angle, are worked as pillars and chambers, where two chambers on the same horizontal level are separated by a pillar of unquarried rock, of a similar thickness to the width of the chamber, in order to support the rock above. In order to maintain the stability of the quarry, chambers below that level are precisely aligned, so that the pillars on one floor are directly below those on the floor above. The upper levels of Moelwyn Quarry were at above ordnance datum (AOD), while Pant Mawr was around AOD. The integrity of the Moelwyn Quarry was ensured by making underground connections to it, and aligning the Pant Mawr chambers and pillars with those of the quarry above. The numbering sequence for the levels in the Moelwyn Quarry was continued downwards through the Pant Mawr Quarry.