The geological history of Cook County, Minnesota includes volcanoes, glaciers, a meteor impact, two-billion-year-old rock, and a gash in the Earth’s crust that spilled molten rock out of the planetary core.

It’s a long story. Here are some highlights.

Geological history of Cook County (abridged)

Some of the oldest exposed rock on the planet underlies Lutsen-Tofte-Schroeder, Grand Marais, the Gunflint Trail and Grand Portage. Lake Superior rests in a formation of ancient Precambrian bedrock known as the Canadian Shield, which is over two billion years old.

Two dramatic geological events shaped the Cook County landscape:

  • A splitting-apart of the Earth’s crust, which caused a system of fissures called the Mid-continent Rift System. The splitting opened the earth’s core, enabling hot molten rock to well up, forming basalt flows that can be seen today along Lake Superior’s North Shore.
  • Ice ages. Starting about 2 million years ago, glaciation and glacial recession carved the Great Lakes, the Boundary Waters Canoe Area Wilderness/BWCAW, and the ten thousand lakes for which Minnesota is known.

The granite cliffs you’ll see while traveling the Gunflint Trail Scenic Byway are part of the Laurentian Divide. Waters on the north side of the divide flow north to Hudson Bay and the Arctic Ocean; waters on the south side flow southeast to Lake Superior, then on the Atlantic Ocean. Be sure to stop at the Laurentian Divide Scenic Overlook (Birch Lake).

Palisades up to 400 feet high tower over many Gunflint Trail lakes, including Pine, Mountain, Clearwater, Watap, Hungry Jack, West Pike, and Rose.

Geology buffs may want to search out the Sudbury Meteor impact layer on the Gunflint Trail. When this meteor crashed into the earth some 1.9 billion years ago, it punched an enormous crater in Sudbury, Ontario, and sent rocks flying all the way to Minnesota. Take a short hike to Magnetic Rock – which is “magnetic”. And don’t forget to look for agates.

For detailed geological information, check out this PDF about the Sudbury Meteor impact layer.

In Lutsen-Tofte-Schroeder and Grand Marais, you’ll marvel at the Sawtooth Mountain Range, born of glacial action and ancient volcanic eruptions. It is so named because the angle and relatively uniform size and spacing of its peaks make it resemble the teeth of a saw.

To enjoy the Sawtooth Range, walk out to the Grand Marais lighthouse and look west toward Lutsen for a great view of the “saw.” Trek some challenging sections of the Superior Hiking Trail. And, in the winter, head to Lutsen Mountains—the biggest, tallest downhill ski area in Minnesota.

Learn more about the geology of Cook County, Minnesota with the Minnesota Outdoor Classroom Field Guide to Geology or by attending a naturalist program.