The principal metal mined in North Wales is lead. Lead is usually found in near-vertical veins up to 4 metres in thickness which were intruded into the fractured limestone, sandstone and chert which forms the higher ground of Flintshire and Denbighshire.
The mineralisation took place between 100 and 200 million years after the rock was deposited, hot mineral rich fluids rising from molten igneous rock at depth exploiting the faults and fractures in the host rock. Other minerals found in conjunction with lead were zinc and silver.
All three were mined, and separated afterwards in the refining processes.
Lead veins were worked initially at outcrop to create a linear ditch feature along the line of the vein. Deeper mining in later years was by tunnels from shafts sunk to intercept the vein at depth, then working upwards in the vein towards the previous workings.
As the veins are near vertical, the outcrops and associated old shaft mounds form near-linear features across the countryside. Lead ore was raised to the surface and smelted in the coalfield, but waste material from the veins was stored on temporary timber supports within the workings which have subsequently decayed and may be liable to collapse.