Originating in the mediaeval Northern March, the Margraviate of Brandenburg grew to become the core of the Kingdom of Prussia, which would later become the Free State of Prussia. The eastern third of historic Brandenburg (Ostbrandenburg/Neumark) was ceded to Poland in 1945.In 1952, the East German government divided Brandenburg among several Bezirke or districts. (See Administrative division of the German Democratic Republic). Most of Brandenburg lay within the Potsdam, Frankfurt (Oder), or Cottbus districts, but parts of the former province passed to the Schwerin, Neubrandenburg and Magdeburg districts (town Havelberg). East Germany relied heavily on lignite (the lowest grade of coal) as an energy source, and lignite strip mines marred areas of southeastern Brandenburg. The industrial towns surrounding Berlin were important to the East German economy, while rural Brandenburg remained mainly agricultural.