Operations
One of the reasons for Aberllefenni Slate Quarry's continued use was the high quality of the slate extracted. There are two major "bands" of slate running parallel to each other through this region of mid-Wales, the "broad vein" and the "narrow vein", the latter of which Aberllefenni extracted. The broad vein is of considerably poorer quality than the narrow vein, and the slate it produces is of little use in roofing slates or polished surfaces. Instead it was used in walls, fences and hardcore. The narrow vein was of much better quality and could be used in much higher quality applications and fetched higher prices.
Slate extracted from the narrow vein at Aberllefenni is deep blue and extremely hard and dense. It resists fine splitting, so most of the mines' product was large cut slabs rather than split roofing slates. Foel Grochan mine consists of eight near-horizontal tunnels at approximately 60 feet (18.3 m) vertical separation. These were bored into the valley side just to the north of the near-vertical narrow vein. Each tunnel connected to a large chamber from which the rock was extracted. These chambers ranged from 100 to 187 feet (30.5 to 57.0 m) to in length with 24 to 30 feet (7.3 to 9.1 m) of rock left between the bottom of one chamber and the top of the next lower chamber. As more slate was extracted, several of the upper chambers were joined vertically to form an extremely large cavern known as Twll Golau which is open at the top.
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