The Black Country is an area of the English West Midlands, covering most of the Metropolitan Boroughs of Dudley, Sandwell, and only some minor parts of Walsall and Wolverhampton. Dudley and Tipton are generally considered to be the centre. It became industrialised in the Industrial Revolution, with coal mines, coking, iron foundries, glass factories, brickworks and steel mills, producing a high level of air pollution. The name dates from the 1840s, and is believed to come from the soot that the heavy industries covered the area in, although the 30-foot-thick coal seam close to the surface is another possible origin. The 14-mile (23 km) road between Wolverhampton and Birmingham was described as "one continuous town" in 1785. Extent The Black Country has no single set of defined boundaries. Some traditionalists define it as "the area where the coal seam comes to the surface – so West Bromwich, Coseley, Oldbury, Blackheath, Cradley Heath, Old Hill, Bilston, Dudley, Tipton, Wednesfield and parts of Halesowen, Wednesbury and Walsall but not Wolverhampton, Stourbridge and Smethwick or what used...