The Petoskey stone was officially designated as Michigan’s state stone in 1965. It is a popular stone recognized by its distinctive, polished surface that reveals an intricate pattern of hexagonal shapes. When dry, the stone often resembles ordinary limestone, but wetting or polishing it makes the pattern beautifully prominent. This preserved natural history tells a long geological story, attracting collectors and casual beachcombers alike.
The Petoskey Stone: A Fossilized Coral
The Petoskey stone is a fossilized remnant of a colonial rugose coral, specifically Hexagonaria percarinata. The characteristic honeycomb appearance is a cross-section of the coral’s structure. Each hexagon represents a skeletal cup, or corallite, that housed an individual coral animal called a polyp. Thin lines radiate outward from a dark center within each unit, marking the polyp’s former mouth.
This ancient organism lived in large colonies, constructing reefs in the shallow sea. Over millions of years, the original organic material was replaced by minerals, primarily calcite, through permineralization. This process turned the skeletal structure into a durable fossil found within the Traverse Group formation in Michigan.
The Devonian Period: Pinpointing the Age
The Petoskey stone’s age is determined by the geological time period during which the coral lived. These fossils date back to the Middle Devonian Period, placing their origin approximately 350 to 400 million years ago. This makes the Petoskey stone significantly older than the first dinosaurs, which appeared much later in the Triassic Period.
Geologists determine this ancient age by studying the rock layers, or strata, where the fossils are found. The specific species, Hexagonaria percarinata, is unique to the Gravel Point Formation, a division of the Devonian-aged Traverse Group. By correlating these specific strata with other rock formations containing index fossils of known ages, scientists confidently assign a date range to the Petoskey stone’s existence within the Paleozoic Era.
From Tropical Reef to Glacial Deposit
During the Devonian Period, the landmass that would eventually become Michigan was situated much closer to the equator. This tropical location meant the region was covered by a warm, shallow, saltwater sea, ideal for Hexagonaria percarinata to thrive and form extensive coral reefs. As tectonic plates shifted, the region moved northward, and the coral reefs were eventually covered and compressed into limestone bedrock.
The stone’s final journey began during the Pleistocene Epoch, approximately two million years ago. Massive continental glaciers advanced and retreated across North America, scraping and eroding the bedrock layers. These sheets of ice plucked the fossilized coral fragments from the Traverse Group rock and transported them across the northern Lower Peninsula of Michigan. The grinding action of the glaciers rounded and polished the fragments, depositing them as glacial till along the shores of the Great Lakes today.