The Ancares river basin includes many Roman gold mines. The Ancares River (also called Cuiña in its upper reaches) is enlarged by the waters of many rivers and streams, such as the Seco, Suertes, until it itself flows into the Cúa. The valley of the Ancares River, and the homonymous Leonese area are part of the complex geological structures found in the western end of the Cantabrian Mounatins (known as the región Astur-Occidental leonesa), which is basically the result of alpine tectonics. It is a mountain landscape in which the orogenic movements and the setting in of the fluvial system have generated a broken landscape. As a result, mountain ranges such as the Ancares or O Caurel have high mountains –approaching 2000 m like the Pico Miravalles– which alternate with deep isolated valleys. A great lithostratigraphic variety has prompted a drastically different erosion in different parts. During the Quaternary, various processes and types of deposits have culminated the area’s morphology:
- Glacial and periglacial phenomena have created cirques at the mountain tops, morraines and abundant Quaternary glacifluvial deposits. A part of the eroded material have been transported by ice, and later by water. This material includes large isolated glacial erratics and striated boulders. Between Pereda de Ancares and Sorbeira there are glacifluvial terraces which extend up to 1 km on both sides of the river. In this part, indeed, the valley is characteristically Ushaped.
- The river system. The upper reaches of the rivers in the basins of the Cúa and Burbia, including the Ancares and its tributaries such as the Seco and the Vega, are almost throughout mountain rivers. In the upper waters of Candín and Ancares, the areas eroded are small, and they have consequently only created small alluvial deposits near the current course of the Ancares River, as well as in some tributaries. The formation of alluvial fans in the forks of rivers is frequent, given the sharp differences in altitude. Only downriver from Candín can actual fluvial sedimentation be witnessed, forming natural terraces.
- The steep slopes formed by Quartzite, or the walls of the cirques and glacial valleys, have seen the accumulation of abundant screes (colluvia).
Under Roman domination this mining area was heavily exploited. The deposits were either hard rock or conglomerates of glacifluvial origin. The mines themselves can be grouped in three sectors: two of the first type and one of the latter.
Roman mines tapping glacifluvial deposits span on the right bank of the river, some 4.5 km between some fields to the north of Pereda de Ancares all the way down to the bridge over the Ancares which lies just downstream from Sorbeira. The mines also extend up both sides of the Seco creek. They are easily discernible through the massive heaps of boulders (murias) which result from the washing of the ore from the glacifluvial conglomerate. Three main areas can be identified here: As Fontes or Los Entralgos is the most southerly one, located between the bridge at Sorbeira and the Teso de Altamira. Although enormous murias up to 8 m high can be found, the forest effectively conceals the rest of the mine. Upriver, Las Moracas is a toponym which refers to the large heaps of boulders also present there, between the Teso de Altamira and the confluence of the Seco into the Ancares. Finally, there is an additional mine of the glacifluvial deposits on the left bank of the Seco creek (Los Castros), as well as on the right bank of the Ancares River (Las Cavadas).
The two ancient mines exploiting hard rock deposits are A Regueira das Meixoncías/ Las Cavanías and Las Labradas. They are both quite close to each other, and they share geological and technical characteristics. They are open-pit mines in which water is used to extract the mineral from the quartz reefs that penetrate the Lower Ordovician (specifically the regional Los Cabos series) quartzite or schist.
Many later transformations have altered these mines and their associated hydraulic networks: tracks, 20th century mines, afforestation and modern water systems.
At A Regueira das Meixoncías/ Las Cavanías the mine is divided in two massive trenches which run down 600 m of mountainside, with a width that varies from 30 to 150 m. The debris cone ejected from these pits covers the left bank of the Seco creek.