Geology - Southern extension
The Superior Upland of Wisconsin and Minnesota is the largest U.S. surface exposure of the ancient (2.6 to 1.6 billion years old) core of the North American continent, known geologically as the Canadian Shield. Even older rocks have been recognized in northwest Canada. Now an area of low topographic relief, these metamorphic rocks once themselves formed mountains- located at the margin of a continent, just as the Appalachian and Rocky Mountain ranges were in their turn. Some of these highly altered rocks have been important sources of iron, copper, and other industrial minerals. - USGS
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