Unlike coal which is found in horizontal seams, metal minerals such as for Lead (Pb) and Copper (Cu) ore are found in long, vertical deposits known as veins. They may be short or up to several miles in length. At the surface they may be only a few feet wide. In further contrast to coal the thickness of a vein may vary substantially from centimetres to metres. This is due to them forming in between existing rocks. The presence of many veins is usually referred to as an orefield.