 The resulting thermal shock cracked the rock, enabling it to be removed by further streams of water from the overhead tanks.The Roman miners used similar methods to work cassiterite deposits in Cornwall and lead ore in the Pennines.The methods had been developed by the Romans in Spain in 25 AD to exploit large alluvial gold deposits, the largest site being at Las Medulas, where seven long aqueducts tapped local rivers and sluiced the deposits.