Schematically speaking, in the southern half of the concessions, the great concentration of pits proves the French term “gruyère” is a fitting definition. In and around Lens, Denain and Bruay-La-Buissière, some pits were barely a couple of kilometres apart! Further to the north, the extraction pits were more scattered. How come there was such a great difference? The coal seam was far from uniform. The real “black gold” was to be found in the south, where the seams were deep and rich in bituminous coal and coking coal, needed for heavy industry, cast iron and steelworks. They were mined intensively, as can be seen from the many collieries and miners’ housing developments. Further to the north, the lower-grade and semi bituminous coals were used for other purposes, long considered less lucrative. These were mined less intensively. In other words, differences at the coal face reflected those on the surface!