The mines at Trigal Mountain and the deposit at A Poza dos Lamocos
The hard rock deposit of Trigal Mountain (San Antolín de IBias), includes several open-cut pits on the mountainside that looks upon the right bank of the Ibias, between Cabo creek to the south, and Vilanova creek to the north.
The technique used included the arrugia (i.e. provoking the collapse of the deep alluvial deposits using water and tunnelling, described by Pliny the Elder). Once the conglomerate had crumbled it was easy to separate ore and gangue, and then grinding the former until it was pulverized. In these latter stages water continued to play a key role because it was used to wash away the tailings from the mining area, and for washing and panning the ore for gold.
All evidence indicates that, apart from the six pits identified, a more selective mining was carried out, as witnessed by the six small trenches (some undoubtedly for prospection purposes) and underground tunnels which were excavated to pursue the richer veins.
The considerable hydraulic network which was necessary to carry out this type of mining can still be identified, albeit fragmentarily: some sections of canals, and deposits. One of the latter has been studied in detail, including a microtopography and a sondage excavation in order to understand its morphology and phases of use and abandonment. This is A Poza dos Lamocos, a piscina or stagnum (terms used by Pliny the Elder himself) of approximately 37 x 20 m. It was built by adapting a slope, and altering some 1700 m2 of total surface. The interior of the deposit occupies 845,13 m2. The topography and excavation have revealed that the maximum depth was 2.7 m, indicating a capacity ranging from 771 to 1085,5 m3.
The deposit was physically constrained by slate outcrops on the northern and southern end. The western side, however, required the construction of a considerable slate wall to keep the water within the deposit. The containment wall, as revealed by surface remains, is 43.5 m long, and it rests primarily on the bedrock, although slate footings were also added to improve the wall’s resistance. In order to build it, the deposit’s bottom was necessarily flattened beforehand. Then rows of large slate slabs were placed, with only some dirt and cobbles used to secure their stability. Both sides of the wall were raised, filled by small stones and dirt. In one of the sondages, the rows of slate appear deformed, indicating that the wall broke at some point, and cracked from top to bottom. The damage had to be repaired using mortar extensively. Al in all, the retaining wall is nearly 3 m tall and up to 2.5 m wide at the base.
This considerable depth has revealed a fascinating stratigraphy which explains the detailed story of how the deposit was constructed, used and abandoned, telling a story that reaches the present.