Although it is part of the state of Michigan, Isle Royale is located in Lake Superior about thirty miles west of the northeastern tip of Minnesota. Veins of pure copper dissected the basalt rock formations. The prehistoric Indians mined these veins using hammerstones to dislodge usable pieces, and then hammering these into implements. Whole areas apart from mining activities appear to have been dedicated to working the copper into tools.
Thousands of loose hammerstones were strewn throughout the mining sites. There were many hammerstones that were notched for handles of some sort, but most of them appear to have been wielded by hand. They ranged from fist-size to as much as twenty pounds. These stones were probably gathered and brought to the island from the western shore of Lake Superior, where the action of the water had smoothed and rounded them.
Mining pits had been burrowed into the rock, some as deep as twenty feet. In some cases there were isolated pockets of copper outcroppings, and then there were veins of copper bearing rock that the Archaic people followed with their primitive mining efforts. As the people worked along these veins they left piles of discarded rock thrown to the side. When the rocks were methodically examined, they were found to contain large quantities of small copper pieces and even larger copper nuggets. This gives us an idea that the copper on Isle Royale was abundant enough for them to be wasteful.
Some of the pits were excavated and revealed pure copper nuggets so large that the prehistoric miners could not get them out of the hole. One such nugget was removed in the 1830s and put on display in Detroit. It weighed 5,720 pounds and clearly showed evidence that material had been removed from its surface. It was found at a depth of sixteen and a half feet deep along with large quantities of charcoal, as fire had been used to soften the copper. Mixed with the charcoal was an abundance of fine copper shavings as a result of hammering on the surface of the nugget. An accepted theory is that the copper on the large surface was beaten sideways into a peak. When there was enough of a protrusion, sharp blows against it would break off a large, usable piece.
The intensity of effort involved in mining the copper on Isle Royale, and other locations, gives some indication of how important the metal had become to the Archaic cultures. The metal quickly spread throughout the northern United States and Canada, possibly accelerating cultural exchanges that otherwise would have taken much longer.
Griffin, James B., Editor, Lake Superior Copper and the Indians: Miscellaneous Studies of Great Lakes Prehistory, Anthropological Papers, No. 17, Museum of Anthropology,University of Michigan, Ann Arbor, 1961.