During the upper Carboniferous period (Namurian, Westfalian and Stephanian) large areas of central western Germany were covered by coastal swamp forests dominated by Lepidodendron und Sigillaria. Periodic marine and fluvial transgressions caused the swamps being regularly buried by siliciclastic material, resulting in up to 5500 m thick successions of alternating organic-rich and clastic-rich sedimentary rock. The organic-rich packages were later subject to coalification producing up to 100 individual coal beds in the area of the river Ruhr. Presumably, initial (private) coal mining started during the Middle Ages, however, with the onset of the Industrial Revolution, commercial mining started and peaked in the 19th until mid of 20th century with more than 120 million tons of coal mined per year. Today, only a few pits are mined sporadically, with the last pit (Zeche Proper Haniel) ceasing service this year.