Michigan Float Copper, Keweenaw Peninsula
Repeated glaciations thousands of years ago altered the geologic deposits of the native copper when glaciers gouged out the Great Lakes. Copper, along with other rocks, gravel, and sand were constantly tumbled and deposited over large areas of the Upper Midwestern United States. These nuggets, sometimes weighing tons, are known as "float copper" because they "floated" in the ice of glaciers after being ripped from the bedrock by the glacial erosion process. The long period of burial since the cessation of glacial movements (ca. 10,000 years ago) oxidized the surface copper. The resulting oxidation products are primarily malachite (a green copper carbonate) and cuprite (a red copper oxide). This uniquely shaped specimen has been partially face polished and coated with a thin coat of lacquer to prevent oxidation.