Las Médulas is a historical site near the town of Ponferrada in the region of El Bierzo (province of León, Castile and León, Spain), which used to be the most important gold mine in the Roman Empire. Las Médulas Cultural Landscape is listed by the UNESCO as one of the World Heritage Sites.
The spectacular landscape of Las Médulas resulted from the Ruina Montium, a Roman mining technique described by Pliny the Elder in 77 AD. The technique employed was a type of hydraulic mining which involved undermining a mountain with large quantities of water. The water was supplied by interbasin transfer. At least seven long aqueducts tapped the streams of the La Cabrera district (where the rainfall in the mountains is relatively high) at a range of altitudes. The same aqueducts were used to wash the extensive gold deposits. The area Hispania Tarraconensis had been invaded in 25 BC by the emperor Augustus. Prior to the Roman conquest the indigenous inhabitants obtained gold from alluvial deposits. Large-scale production did not begin until the second half of the 1st century AD.
Description in Pliny the Elder’s Natural History
“What happens is far beyond the work of giants. The mountains are bored with corridors and galleries made by lamplight with a duration that is used to measure the shifts. For months, the miners cannot see the sunlight and many of them die inside the tunnels. This type of mine has been given the name of Ruina Montium. The cracks made in the entrails of the stone are so dangerous that it would be easier to find purpurine or pearls at the bottom of the sea than make scars in the rock. How dangerous we have made the Earth!“
The description could well have applied to Las Medulas. Since Pliny was a Procurator in the region in 74 AD, it is highly likely that he saw mining operations for himself, and his text reads like aneye-witness report. He also describes the methods used to wash the ores using smaller streams on riffle tables to enable the heavy gold particles to be collected. Detailed discussion of the methods of underground mining follows, once the alluvial placer deposits had been exhausted and the mother lode sought and discovered. Many such deep mines have been found in the mountains around Las Medulas. Mining would start with the building of aqueducts and tanks above the mineral veins, and a method called hushing used to expose the veins under the overburden. The remains of such a system have been well studied at Dolaucothi in South Wales. Opencast methods would be pursued by fire-setting, which involved building fires against the rock and quenching with water. The weakened rock could then be attacked mechanically and the debris swept away by waves of water. Only when all opencast work was uneconomical would the vein be pursued bytunnelling and stoping. Pliny also stated that 20,000 Roman pounds of gold were extracted each year. The exploitation, involving 60,000 free workers, brought 5,000,000 Roman pounds (1,650,000 kg) in 250 years.
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