The Canadian Shield was formed over billions of years as a stable core or craton of the North American continent. It is composed of ancient rocks that date back from about 4.28 billion to 2.5 billion years ago. The Shield formed through a complex process of volcanic activity, mountain building (orogeny), and the amalgamation of smaller continental fragments and terranes between about 2.5 and 1.25 billion years ago. The area was once covered by towering mountain ranges, higher than present-day Everest, but these were gradually eroded down to a rolling plateau over millions of years. The Shield contains greenstone belts from volcanic rock and granitic rocks characteristic of the Archean continental crust. The region was also shaped by glaciations during the Pleistocene Epoch, which scraped the land bare and created the present landscape of rocky hills and thousands of lakes. The Canadian Shield is the Earth's largest area of exposed Precambrian rock and the nucleus around which the North American continent was built.