The Penokees Are A Geologic Gem

University Place: The Complex Story Behind 'Some Of The Oldest Rocks In Wisconsin'
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Corrigan's Lookout, off of Highway 122 near Saxon, offers a sweeping view of the Penokees.

In a state where the landscape tends towards low-lying swamps, flat fields and rolling hills, northern Wisconsin's Penokee Range of mountains are a dramatic outlier. Though their elevation is relatively modest — the Penokees are often just called hills — the range stands out. Billions of years of geologic processes, a cataclysmic meteor impact and changes in Earth's atmosphere all contributed to shaping a land with striking visual and physical properties.

The Penokees consist of two elevated ridges that run about 50 miles through Ashland and Iron counties, and continue on for about another 50 miles in the Upper Peninsula of Michigan, where they're referred to as the Gogebic Range. Like seven similar outcrops around Lake Superior, the Penokee-Gogebic Range has plentiful deposits of iron and other minerals.

Iron deposits create a variety of dramatic banded formations in the Penokees.
Tom Fitz/Northland College

In 2011, this fascinating geology collided with environmental concerns and renewed a statewide debate over mining. A company called Gogebic Taconite began seeking approval from the Wisconsin Department of Natural Resources to open a new iron ore mine in the Penokees. After a long and politically contentious process that saw the company helping legislators craft a new iron mining bill, with Wisconsin's Native American tribes and environmental advocates sounding alarms about the proposed mine's impact on water quality, Gogebic Taconite backed out, citing concerns about federal wetland regulations.

Tom Fitz, an associate professor of geoscience at Northland College, was involved in the mining debate, noting the presence of small asbestos-like fibers in the Penokees that could damage people's lungs if disturbed. However, Fitz has been studying the range since long before the Gogebic Taconite controversy, and thinks about its geology in terms of a much longer time scale.

In a July 27, 2016, talk hosted by the Lafayette County 4-H club, recorded for Wisconsin Public Television's University Place, Fitz delved into how the Penokees — "some of the oldest rocks in Wisconsin" — took shape. He explored the processes that formed the complex layers of rock making up the range, detailed the benefits the region provides, and discussed the difficulty of balancing different human and environmental needs.

Fitz pointed out a lot of the beauty simply present in the region's geology. He even once collaborated with a photographer on an artistic photo series inspired by the rocks of the Penokees. Taken with how vividly the region's geology illustrates the history of Earth, Fitz said, "it is also beautiful in itself, just to see it."

Key facts

  • The Penokee Range consists of two 400-foot ridges that run roughly southwest to northeast alongside each other, one to the north and one to the south.

  • The three main types of rock in the range were all formed eons ago. About 2.7 billion years ago, the continents as they currently exist were only beginning to take form, and the area that would become northern Wisconsin was underwater. Eruptions from submarine volcanoes created a layer of rock on the ocean floor. Much of this rock was eroded away over the next few hundred million years, but some of it remains as the foundation of the current-day Penokees.

  • About 2.5 billion years ago, cyanobacteria began carrying out photosynthesis on a large scale, releasing increasing amounts of oxygen into the atmosphere. During this process, the oceans were rich in dissolved iron, and as this combined with the newly available oxygen, the two elements formed deposits of iron oxide, interspersed with layers of other rock, and are called banded iron formations. In the Penokees, this is known as the "Ironwood" formation, and visitors can see it illustrated dramatically in the banded (or striped) cross-sections of exposed rock in the area. The formation of these layers likely wrapped up about 1.85 billion years ago.

  • Also about 1.85 billion years ago, a meteor more than six miles in diameter slammed into what is now Ontario to create the Sudbury Basin. This impact created a 10-foot layer of broken rock that sits between the Penokees' Ironwood formation and the soil above.

  • About 1.1 billion years ago, what is now the North American continent began to divide, instigating massive lava flows that would form the basalt that makes up much of the range's northern ridge.

  • These geologic processes created an estimated 2.7 billion tons of economically viable iron ore in the Penokees. Much of the iron used in the United States has come from the eight "iron ranges" around Lake Superior, especially the Mesabi Range in Minnesota. There is also a small amount of gold mixed into the gneiss formations in the range.

  • Taconite, a low-grade form of ore found in the banded iron formation, is the focus on renewed interest in iron mining in the Penokees.

  • The sheer heat of geologic processes over billions of years created long, skinny mineral formations known as amphibole fibers — a form of asbestos — in some parts of the Penokee Range. If disturbed, this mineral can become airborne as needle-like fibers that can be inhaled and cause a form of cancer called mesothelioma. The range also contains iron sulfide that, when ground up and combined with water in some mining processes, can create acidic drainage. Both substances contribute to environmental concerns about mining in the Penokees.

Key quotes

  • On envisioning long-term geological processes: "When we're talking about geologic stories, we have to extend our thinking of time into deep time."

  • On the magnetic properties of the Penokees: "There aren't many places in the world where you can go up to a rock and throw a magnet and it sticks, and the Penokee Range is one of those places … it's also one of the few places on earth where your compass doesn't necessarily point north, which can make navigation in those words very challenging."

  • On the amount of gold present: "There's just enough to keep people interested. Maybe more."

  • On amphibole fibers: "In some places the rock is mostly this mineral … the concern there is that if these things become airborne and get lodged in your lungs, they stay there indefinitely and in some people they cause disease … Every place where rocks rich in this mineral have been mined, people have died of mesothelioma."

  • On the environmental challenges surrounding the Penokees: "We all use resources and we have these competing needs. We need iron, we need clean water, we need clean air, we need wilderness space, we need forest products. We have all of these competing demands and we have an increasing population."
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