Most Michigan beaches have fossils. Here’s how to find them

Today’s MI Environment, by Evin Maguire, senior geologist in the Materials Management Division at the Michigan Department of Environment, Great Lakes, and Energy (EGLE), is the latest in the Rock and Mineral Identification series.

Every Michigander knows about Petoskey stones – rare treasures that can be found on countless beaches on Michigan’s lower peninsula. Some more savvy rockhounds may even know that they are fossil corals in the genus Hexagonaria, dating back to the Devonian Period some 350 million years ago.

While most common on Up North beaches, fossils -- any preserved remains, impression, or trace of ancient life from a past geological age -- can also be found farther south, with known collecting localities from the Sleeping Bear Dunes to Fort Gratiot County Park.

They can also be found inland and have been uncovered in many sand and gravel mines across the state.

While Petoskey stones may be among the most prized fossil finds on Michigan beaches, there are a wealth of other fascinating fossils to be found while combing for ancient treasures.

If you look at a geologic bedrock map of Michigan, you will immediately notice a distinct “bullseye” pattern. This is what geologists refer to as the Michigan Basin, and it’s one of the most unique geologic structures on the planet. It covers the entire lower peninsula, and roughly half of the upper peninsula.

The edge of the basin is roughly defined by the ~541to 486 million year old (Cambrian age) Munising Formation, and at its center is the ~300 million year old (upper Pennsylvanian age) “Red Beds” of the Pewamo Formation.

Rocks get younger toward the center of the basin, making the youngest bedrock in Michigan over 300 million years old! The Michigan Basin was formed by a combination of tectonic forces, and possible reactivation of geologic faults of the 1.1 billion-year-old failed “Mid-Continent Rift,” which lies at the center of the basin, buried under more than 4 kilometers of rock and sediment.

Given these brutal past couple winters in Michigan, it may be difficult to imagine that for many millions of years, the state was covered by vast, shallow tropical seas that inundated the low-laying Michigan Basin. It is from these ancient seas that (almost) all of the fossils you can find on Michigan’s beaches originate.