Geology of the Mines

During the period of time that geologists call the Carboniferous era, approximately 340 million to 280 million years ago, North Wales, like many other parts of Britain lay underneath a shallow tropical sea, with similar conditions to present day Bermuda or Florida. Skeletons of plankton, and shells settled on the bottom of the sea and were broken up by the motion of the waves into tiny particles. Plankton skeletons and shells are made almost completely out of lime so over millions of years, beds of lime, hundreds of feet thick accumulated on the sea bed, solidified, and turned into limestone.
 

 

1) Dead animals sink to the sea bed and break up into tiny particles

              

2) The layers of sediment turn into rock, some of the skeletons and shells remain complete and harden to form fossils

 

3) The rock is folded and parts are eroded

  

4) The fossils are exposed on the surface. Many can be seen on the Great Orme today

 

Sometime after this, most likely between 280 million and 200 million years ago earth movements put a great deal of pressure on the limestone and caused it to crack. Many of these cracks would have been very small, but some of them would have penetrated deep into the earth’s crust. Within the earth’s crust there are pockets of boiling hot minerals, liquids and gases which are under immense pressure. These would have forced their way towards the surface through the cracks; some of the liquids and gases would have dissolved areas of limestone around the cracks to form cavities. The minerals, which in this case were copper minerals would fill these cavities, cool down, solidify and form veins of copper ore.
 

 

Fissures or Faults appeared in the rock
due to the pressure it was under
Mineral rich magma from the earth’s core, under 
great pressure, forces its way up through the crack

 
The veins of copper are wider when going through various layers
of rock as some are more soluble than others.

During this time the limestone rock surrounding the veins would have undergone a chemical change. Some of the lime in the limestone would have been replaced by magnesium turning it into magnesium limestone or Dolomite.

This Dolomite would have been slightly softer than the limestone, and combined with the fact that the copper ores were exposed on the surface would have provided favourable conditions for early miners.