Beachgoer’s Guide to Lake Michigan Fossils and Rocks

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Various rocks sorted out by type on a piece of white pastic

When you think of Lake Michigan, does fossil hunting come to mind? Many people may not be aware that the beaches of Lake Michigan can be a hot spot for fossils hunters. Aside from fossils, Lake Michigan beaches are also home to many fascinating rocks. Take a look at some of the more common rocks and fossils that can be spotted around the lake (particularly on the Michigan side):

Concrete: Concrete is a man-made product. This piece has been eroded and polished by the surf and sand and can be difficult to tell apart from a conglomerate, a sedimentary rock.

Zebra mussels: Zebra mussels are small striped shells from a modern bivalve that has invaded North America.

Slag: Slag is often black, shiny, and sometimes almost iridescent. It was probably produced from burning coal on ships or from smelting iron.

Granite: Granite is an igneous rock that formed deep underground and is abundant in northern Wisconsin, the Upper Peninsula of Michigan, and Ontario. The red or pink mineral in granite is potassium feldspar.

Crinoids: Crinoid fossils look like small discs with holes in their centers, like Cheerios. They are from the stems of an animal that looks a little like a flower, but is really a relative of the starfish. The discs were stacked together to form a long stalk that attaches the animal to the sea floor.

Petoskey Stones: Petoskey stones are an extinct fossil coral that lived 380 million years ago. The Petoskey stone also happens to be the official state stone of Michigan. These corals are called colonial rugose corals and lived in the Devonian Period. The fossil corals have eroded out of the rock and are polished by the waves and sand.

Jasper: Jasper is really an orange to tan color variety of chert, which is microcrystalline quartz.

Stromatoporids: Stromatoporids are an extinct calcareous or stony sponge. These were the dominant reef builders in the Silurian Period 430 million years ago.

Honeycomb Corals: Honeycomb corals are another type of extinct coral called favositid corals. The openings for the coral polyps are much smaller than in Petoskey stones and look like a lace pattern draped over the rock.

Syringoporoids: Organ pipe corals, or syringoporoid corals, is yet another extinct group of coral that can be spotted on the beach.

Please make sure to follow all rules and regulations when collecting fossils and geologic specimens, and respect the rights of property owners.