The Baltic Shield (or Fennoscandian Shield) is a segment of the Earth's crust belonging to the East European Craton, representing some of the oldest exposed rock in Europe. This massive geological formation covers most of Fennoscandia and northwestern Russia. The rocks here, primarily granites and gneisses, date back as far as 3.5 billion years. During the last Ice Age, massive glaciers scraped across the shield, stripping away the topsoil and carving out the thousands of lakes and fjords that define the modern Scandinavian landscape. The weight of the ice was so great that the land actually depressed; since the ice melted, the land has been slowly 'rebounding' in a process called post-glacial uplift. This region is a major source of metals like iron, nickel, and copper, with some of the world's largest underground mines located in Kiruna, Sweden.