The Lake Superior region (Wisconsin, the Upper Peninsula of Michigan, and Minnesota) contains 41 Precambrian crystalline (medium- to coarse-grained igneous and high-grade mstamorphic) rock complexes comprising 64 individual but related rock bodies with known surface exposures. Each complex has a map area greater than 78 km2. About 54% of the rock complexes have areas of up to 500 km2, 15% fall between 500 km2 and 1000 km2, 19% lie between 1000 km2 and 2500 km2, and 12% are over 2500 km2. Crystalline rocks of the region vary widely in composition, but they are predominantly granitic.