South of Isle Royale, the enormous Keweenaw Peninsula juts out from the northern coast of the Upper Peninsula. Here, you’ll find several former mining towns, a few inland lakes, and numerous historic attractions that comprise the Keweenaw National Historical Park.

The Keweenaw is home to some of the Midwest’s most distinctive geology and oldest exposed rock. Part of the Precambrian Canadian Shield, the peninsula was created more than two billion years ago by spewing volcanoes and colliding continents. Much later, about 1.2 billion years ago, the hardened crust broke apart. More lava began seeping out through fissures, forming bedrock called basalt. For hundreds of years, the basalt piled up thicker and thicker; groundwater percolated in, too, filling the bubbles and cracks with minerals, creating the Keweenaw’s vast deposits of pure copper.

Eventually, the basalt layers sank, forming a basin surrounded by tilted, uplifted rock. These high spots today are visible as the Keweenaw Mountain Range, the spine that runs the length of the peninsula and across Isle Royale. Consequently, the basalt found throughout the Keweenaw is believed to be among the oldest—perhaps the oldest—exposed volcanic rock on earth.

Though the Keweenaw’s variety of minerals is not particularly vast, it contains some unusual gemstones in substantial quantity. Rock hounds seek out three in particular: porcelaneous datolite, mohawkite, and chlorastrolite. Chlorastrolite is more commonly known as the Isle Royale greenstone, a tortoiseshell-patterned rock rarely found elsewhere in the world. The peninsula’s beaches are also a good place to hunt for agates, thomsonite, epidote, zeolites, red feldspar, and others. Area mineral shops are excellent sources for guidebooks and hunting tips.

Getting to the Keweenaw Peninsula

Bus service is spotty in the U.P. in general, and in the Keweenaw in particular. While you can certainly fly into the Houghton County Memorial Airport (CMX) (Houghton, 906/482-3970, www.houghtoncounty.org), it can be an expensive way to go.

For those traveling by car, U.S. 41 and M-26 are the main routes to the peninsula. Many secondary roads in the Keweenaw are dirt or gravel. For old logging roads and other questionable routes, a four-wheel-drive vehicle is highly recommended.