The first important mines appeared in the 1750s, in the valleys of the rivers Ruhr, Inde and Wurm where coal seams outcropped and horizontal adit mining was possible. In 1782 the Krupp family began operations near Essen. After 1815 entrepreneurs in the Ruhr Area, which then became part of Prussia took advantage of the tariff zone (Zollverein) to open new mines and associated iron smelters. New railways were built by British engineers around 1850. Numerous small industrial centres sprang up, focused on ironworks, using local coal. The iron and steel works typically bought mines, and erected coking ovens to supply their own requirements in coke and gas. These integrated coal-iron firms ("Huettenzechen") became numerous after 1854; after 1900 they became mixed firms called "Konzern."