Mendip Hills In some areas the limestone and the dolomitic conglomerate have been mineralised with lead and zinc ores. From the time of Roman Britain until 1908, the hills were an important source of lead. These areas were the centre of a major mining industry in the past and this is reflected in areas of contaminated rough ground known locally as "gruffy". The word "gruffy" is thought to derive from the grooves that were formed where the lead ore was extracted from veins near the surface. Other commodities obtained included calamine (zinc ore), manganese, iron, copper and baryte. The eastern area reaches
Pitinga mine By 1990 the mine directly employed 2,000 people, and indirectly perhaps as many as 40,000. Trees were cleared, the river diverted and dams and ponds built before the alluvium deposits were processed. Bucketwheel excavator dredges extracted material rich in cassiterite, which was pumped through pipes to floating processing plants that separated the tin ore. The cassiterite is concentrated on site, then shipped to Mamoré Mineração e Metalurgia in the southern state of São Paulo for casting. Columbite ore is also mined, concentrated and processed on site for use in production of FeNbTa metallic alloy. Up to 1990 a total of