Pliny 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 Médulas. 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.