Coal seams are the compressed fossilised remains of tropical forests, which grew 325 million years ago when Europe was part of a larger landmass at equatorial latitudes. The present day Amazon or Niger deltas are comparable environments.

Whilst the coal is the fossilised vegetation, the sandstones and shales separating the coal seams represent the sand banks and muds which supported and buried these forests as a result of oscillations of sea level, and they frequently contain fossilised plant fragments.

The geological succession in North Wales contains over 30 coal seams greater than 300mm thick within about 500 metres of Coal Measure strata, with coal seams rarely separated from one another by more than 30 metres.

Not all coal seams are thick enough or of sufficient quality to justify mining.

Those seams that have been mined were given names, which reflected their quality (Brassey Coal, Stone Coal, Crank Coal, etc), their thickness (Five Yard Coal, Main Coal, etc), or locality (Hollin Coal, Nant Coal, etc). In most areas of North Wales the coal seams dip at gradients of between 1 in 3 and 1 in 10, usually to the north-east in Flintshire and the east in Wrexham and Denbighshire.

Other minerals were worked on a small scale, frequently in conjunction with coal. These were iron, especially near Acrefair and Brymbo, fireclay in Rhos and Buckley, and sandstone near Mold. The mining techniques of these minerals were similar to that of coal.

The earliest recorded coal mining in North Wales was in Ewloe in the 14th century, and by the 15th century there was coal mining in Mostyn, Nant Mawr Buckley, and Rhos. This early mining was by digging at the coal outcrop and subsequently excavating shallow shafts and bell pits down to a depth of not over 10 metres.

Below this depth mining was by pillar and stall techniques, whereby a grid of tunnels were driven to the royyalty boundaries in the coal seam, with pillars of unworked coal used to support the workings.  From the Victorian period these pillars were usually taken out as the final stage of mining in the seam.

Until the rise of capitalism and the introduction of Limited Liability Companies in the mid 18th century, the majority of mines were shallow and of limited lateral extent, depending on hand labour by a small group of workers with perhaps a horse used to help the men, women and children over the age of 5 haul the coal in baskets.