Smeltmills were water-powered mills used to smelt lead or other metals. The Roman lead smelting has led to evidence of global pollution. Greenland ice cores from 500 BCE to 300 CE show measurably elevated lead content in the atmosphere. Researchers studying an ice core from Colle Gnifetti, in the Swiss part of the Monte Rosa massif, have found that higher historical European airborne lead pollution levels are associated with changes in the monetary system from gold to silver from the year 640 CE, with the principal source likely to be the Melle mines in France. Later airborne pollution, between the years 1170-1216 CE, correlates even more strongly with contemporaneous records of lead and silver production from mines in the Peak District of England, at levels similar to those seen in the Industrial Revolution.