Here is an interesting and unique example of the copper "skulls" found in the Keweenaw Peninsula. Some of the copper deposits in the Keweenaw Peninsula are in conglomerate beds. Sometimes, through geological processes, cobbles in the conglomerates are partially replaced by native copper. The same processes cause the cobbles and matrix to deteriorate and what remains is a cast of a conglomerate cobble. Here is an example that is, in fact, TWO replaced cobbles, side by side. Excellent and unique!
