The Laurentian Plateau, a U-shaped region of ancient rock, the nucleus of North America, stretching north from the Great Lakes to the Arctic Ocean, is also known as the Canadian Shield. All magnetic compasses of the world point here as the location of magnetic north (Northeastern Canada). Covering more than half of Canada, it also includes most of Greenland and extends into the United States as the Adirondack Mountains and the Superior Highlands. The first part of North America to be elevated above sea level, it has stayed almost wholly untouched by many encroachments of the sea upon the continent. Plateaus are widespread, and together with enclosed basins they cover about 45 percent of the Earth's land surface...