Float copper refers to naturally occurring masses of copper metal that have been moved from their original bedrock source by geological forces. This material represents a rare form of pure metal found in nature, contrasting with the much more common copper ores that must be chemically processed. The term is a historical designation used by geologists and prospectors to describe pieces found loose, “floating” in the soil or glacial deposits, far from their original veins. These erratics remain prized specimens today.
The Geological Identity of Float Copper
Float copper is composed of native copper, meaning it is pure elemental copper (Cu), not a compound or sulfide requiring smelting. This purity, often exceeding 95%, is a unique geological feature of the largest native copper deposits, such as those found around the Lake Superior region. The original copper was deposited within fissures, veins, and porous conglomerate rocks of ancient lava flows, forming large masses known as lodes.
The material is distinguished by its high density and characteristic malleability. Before transport, the copper often formed irregular, branching structures within the rock. After being moved, these pieces are frequently found with a distinctive polished or rounded appearance due to abrasion during their journey.
The surface often develops a bright green to black alteration crust, primarily consisting of minerals like malachite and cuprite, due to oxidation from exposure. This oxidized layer contrasts sharply with the brilliant, reddish-orange metallic luster revealed on a fresh or polished surface. Pieces range in size from small nuggets to massive boulders, like the famous Ontonagon Boulder, which weighs over 3,700 pounds.
How Glacial Activity Creates “Float”
The “float” aspect is a direct result of massive continental glaciation that covered large parts of North America during the Pleistocene Epoch. As the immense ice sheets advanced, they scraped and plucked pieces of the dense, native copper from the exposed bedrock source, such as the Midcontinent Rift System. This process entrained the copper masses within the moving glacial ice and debris.
The copper was transported by the glacier, often southward, mixed with less dense rocks and sediments. During transport, the malleable copper was repeatedly tumbled and abraded against surrounding materials, smoothing and rounding its edges. This action sometimes left glacial striations on the copper’s surface or bent the edges upon collision with larger rocks.
When the glaciers retreated and melted, the copper masses were deposited far from their point of origin within the glacial till, moraines, and outwash plains. This dispersed the float copper across the upper Midwest, including Wisconsin, Illinois, and Indiana, creating a geographical trail pointing back to the original lode deposits.
Archaeological Importance and Use
The existence of readily available float copper fundamentally shaped the history of early metalworking in North America. Because the material was nearly pure metal, it could be used by early human populations without the complex knowledge of extracting metal from ore through high-heat smelting. This purity allowed for cold-working, where the copper was hammered into shapes directly.
The people of the Archaic period, centered around the western Great Lakes region, utilized this resource to create a variety of tools, weapons, and ornaments. This tradition, known as the Old Copper Complex, dates back as early as 6,000 B.C., making it one of the oldest metallurgical traditions globally. They crafted utilitarian items such as socketed spear points, knives, axes, and awls, which were more durable than their stone counterparts.
To overcome the brittleness caused by extensive hammering, artisans learned to anneal the copper by heating and rapidly cooling it, allowing them to continue shaping the material. The finished artifacts became valued trade goods, establishing extensive trade networks that moved the metal far from the Great Lakes source region. Although surface collection was likely the first method of acquisition, as demand grew, these early societies began to mine the copper directly from surface-exposed veins.