The Médulas de Caldesiños are a secondary type of gold deposit, i.e. the result of primary rock erosion and subsequent dragging and deposit by river currents. This deposit formed part of the auriferous territories exploited by the Roman Empire in Galicia. Pliny the Younger, an, important Roman magistrate who administered some Galician mines, claimed that at different times they reached 10% of the total empire income. The arrival of the Romans meant the introduction of more advanced exploitation systems than the technique used until then by the Astures, centred on the sifting of river sands. This system used in Caldesiños and described by Pliny is an extensive method called “ruina-montium” consisting of channelling water from where rivers began transporting it to the tanks “piscinae”, located at the top of the mountain until filled. Next the water stored was released and on passing through the open mountain galleries the first material erosion was achieved. The hydraulic force dragged the mass to the wash channels called “agogae”, where the largest examples were eliminated before entering the channels. The finer stones flowed through the evacuation channels to the washing queues where they were selected.
Exploitation of the Caldesiños “médulas” or “borreas” led to a landscape cut-up like a quarter of an orange and abandoned due to the little profitability of the extractions.