 The volcanics and more recent sedimentary rocks were once thought to be underlain by the Duluth Complex across Lake Superior to Wisconsin, where gabbro formations also exist.The Duluth Complex was considered to be a giant lopolith, a lens-shaped structure depressed in the center, connecting gabbro exposures on opposite sides of the lake, but now is recognized to extend only a few kilometers south of Superior's North Shore.The Precambrian bedrock of the Duluth Complex and the North Shore Volcanics are not buried beneath layers of later sedimentary rock, as is common further south; much of this bedrock is close to or at the surface.