Narrow Vein Mudstone Formation

Aberllefenni quarry is the collective name of three slate quarries, Foel Grochan, Hen Gloddfa (also known as Hen Chwarel) and Ceunant Ddu, located in Cwm Hengae, just to the west of Aberllefenni, Gwynedd, North Wales. It was the longest continually operated slate mine in the world until its closure in 2003. Foel Grochan is the quarry on the north side of the valley, facing Ceunant Ddu and Hen Gloddfa on the south; all three were worked as a single concern throughout their history. Rock was mainly extracted underground, though all three quarries had open pits as well.

Aberllefenni is a village in the south of Gwynedd, Wales. It lies in the historic county of Merionethshire/Sir Feirionnydd, in the valley of the Afon Dulas, and in the Community of Corris.

Cwm Hengae is a valley that runs roughly north-west from the village of Aberllefenni in Wales. The Afon Llefenni river runs along the valley and several slate quarries occupy the valley sides. The Roman road Sarn Helen ran along the valley, connecting Aberllefenni with Dolgellau.

Cambergi quarry (also known as Wenallt quarry) was a slate quarry in Cwm Hengae, west of Aberllefenni in North Wales. It operated between 1873 and about 1895.

Mynydd Cambergi or Mynydd Cam-bergi is a mountain in southern Snowdonia, Wales. It is a summit on a long ridge between Mynydd y Waun to the north and Foel Grochan to the east. The mountain forms the north-east side of Cwm Hengae and sits about 0.5 miles (0.80 km) west of the village of Aberangell. Near the summit of the mountain lies the open pit of Cambergi quarry, which was worked from the 1870s to the 1890s.

Cymerau quarry was a slate quarry served by the Ratgoed Tramway, a horse-worked section of the Corris Railway. It is located about half a mile north of Aberllefenni in Merioneth (now Gwynedd), North Wales, on the eastern side of the isolated Cwm Ceiswyn. It worked the Narrow Vein, the highest-quality slate vein in the Abercorris Group.

The Nant Ceiswyn (sometimes known as the Afon Ceiswyn) is a small river in Mid Wales. It flows from the northern flank of Mynydd Dolgoed down to Pont Cymerau, north-east of Aberllefenni. Here it joins the Nant Llwydiarth to form the Afon Dulas that flows south to the Afon Dyfi. There was an ancient bridge at Pont Cymerau.The valley of the river is often known as Cwm Ratgoed (or Ralltgoed), after the Ratgoed slate quarry about two miles north of Pont Cymerau. The quarry was connected to the Corris Railway at Aberllefenni by the horse-worked Ratgoed Tramway, which crossed the river as it passes Cymerau Quarry.To the east of the valley is Mynydd Ceiswyn, part of the chain of ridges running southwards from Maesglase.

The Nant Llwydiarth is a small river in Mid Wales. It flows from the southern flank of Mynydd Hendre-ddu down to Pont Cymerau, north-east of Aberllefenni. Here it joins the Nant Ceiswyn to form the Afon Dulas that flows south to the Afon Dyfi. There was an ancient bridge at Pont Cymerau.

Garneddwen (also known as Garnedd-Wen; English: white cairn) is a hamlet in the south of the county of Gwynedd, Wales. It lies in the historic county of Merionethshire/Sir Feirionnydd, in the valley of the Afon Dulas. It consists primarily of a single row of terraced houses, built for the workers at Aberllefenni Slate Quarry. The hamlet was named after a large cairn ("carnedd" in Welsh) that was to be found in a field below the farm of the same name up to Victorian times.

The Abercwmeiddaw quarry was a slate quarry that operated between the 1840s and 1938. It was located at Corris Uchaf about 5 miles (8.0 km) north of Machynlleth, in Gwynedd, north-west Wales. The quarry was connected to the Corris Railway via the Upper Corris Tramway which carried its products to the Cambrian Railways at Machynlleth for distribution.

The Broad Vein Mudstone Formation (commonly known as the Broad Vein, historically known as the Red Vein and in Welsh as Y Faen Goch) is an Ordovician lithostratigraphic group (a sequence of rock strata) in Mid Wales. The rock of the formation is silty mudstone, intensely bioturbated in places. It varies in colour from a pale to a medium blue. This formation has been commercially quarried as slate in several locations along its length. The formation is between 400 metres (1,300 ft) and 560 metres (1,840 ft) thick and runs from Dinas Mawddwy south-west to Cardigan Bay at Tywyn.

The Abercorris quarry (also known as Cwmodyn quarry) was a slate quarry worked between the mid-1840s and the early 1950s. It was located at Corris Uchaf about 5 miles north of Machynlleth, in Gwynedd, north-west Wales. The quarry was connected to the Corris Railway via the Upper Corris Tramway which carried its products to the Cambrian Railways at Machynlleth for distribution. It worked the Narrow Vein.