The Gogebic and Trap Ranges form two prominent ridges in Iron and Ashland Counties in northern Wisconsin. Both ridges are composed of rock types that are more resistant to erosion than the rock that underlies the valley separating the ridges. Recent glacial deposits cover the valley and parts of the ridges.
The southern ridge, the Gogebic Range, contains iron-rich rock that is approximately 1.9 billion years old; it was mined for iron ore beginning in the 1880s. Soft iron ore was initially removed from shallow mines that have collapsed over time. In the 1920s, harder, high-grade iron ore was mined from depths as great as 5,200 feet. This ore was once a staple of the steel industry in the United States and drew many settlers to northern Wisconsin. Although mining continues in the Upper Peninsula of Michigan and in Minnesota, the industry went bust in Wisconsin in 1964, and the final shipment left the Gogebic Range in 1965.
The northern ridge, the Trap Range, is distinctly different in composition from the southern ridge; it is younger volcanic rock, consisting primarily of basaltic-lava flows that are approximately 1.1 billion years old. These types of flows are present mainly in the subsurface for nearly 1,200 miles as they gently arc from Lake Superior southwest to Minnesota, Iowa, Nebraska, and Kansas and southeast to Ontario, Michigan, and Ohio.
Formation of the Trap Range: Around 1.1 billion years ago, beneath what is now the Upper Midwest, the mantle began to push up and pull apart the crust, nearly tearing the continent in half. Eruptions shook the region, and lava poured out onto the Earth’s surface, eventually forming the Trap Range of northwestern Wisconsin.