Situated within Lower Silurian Gwestyn formation and Ordovician Van formation shales and mudstones. The main vein strikes northeast-southwest with a northern branch, and mineralisation includes galena and zinc.

Became the most productive mine in Great Britain, marked by extreme richness and extent of the ore body. An unlikely discovery at the eastern extremity of the West Mongomery mining field. The mine was of great importance to trade and commerce in Llanidloes.

The extreme width of the lode meant that as well as deads being used to pack the stopes, stone was also trammed in from a nearby quarry.

Workings include five shafts, three adits, and levels, with early trial adits located at SN 9420 8810. Three inclines were built to connect Seahams shaft to the dressing floors—one for hauling coal to fuel the engine and others for transporting ore to the crushers. An additional incline linked the slimes settling tanks to the slimes dump. A tramway embankment carried stone from a nearby quarry to Seahams shaft. The railway built in 1871 for ore and passenger transport still largely survives.

Power was initially supplied by the 50ft x 4ft Mary Emma waterwheel, installed in 1866 to pump the Old Engine shaft. When the Blencowe Consols Engine was introduced in 1875, the wheel was repurposed for crushing. Remains of the waterwheel pit are still visible. By around 1890, as many as thirteen engines were operating at the site for pumping, winding, processing, and compressing. A gas producer was added in 1916, and its foundations were rediscovered in 1992 during archaeological excavations. Also uncovered were the halvans plant engine and an engine base at the foot of the stone piers, both designated for preservation.

Between 1865 and 1871, a large dressing and crushing plant was built, comprising two crusher houses, stamps, buddles, jiggers, and slime pits. A halvans mill, added in 1876, reprocessed spoil tips using a unique set of steam-driven stamps. A mineral separation brine plant, constructed in 1912, later served as a paintworks in the 1930s.

Other features on-site included a sawmill, coalhouses, a carpenter’s shop, a mine office, and a loading bay or surge bin for ore extracted from the main adit. Water was diverted from Llyn Y Fan through the dressing area via the Ceryst and other culverts, while a large dam (SN 9280 8780) was built in the valley to the west to supply water to the dressing floors.

Many buildings in the nearby village were once mining accommodations or administrative offices. The miners’ chapel and its associated library can still be seen at SO 9492 8770.

The preservation of structures on the dressing floors is generally good, particularly where they have been deeply buried under processing waste.

96,739 tons lead ore, 28,424 tons blende.