geographic region in northern Wisconsin, Minnesota, and Michigan, U.S., lying south and west of Lake Superior. A southern extension of the Canadian Shield of ancient mountain ranges, it is composed of crystalline rock with little overlying soil. Pleistocene glaciation (the southward movement of ice that began approximately 2,500,000 years ago) scraped most of the land bare. Agriculture is thus limited, but the region has yielded much iron ore and is ruggedly scenic.