Coral on Lake Michigan
Photo by Susannah
While washing dishes at a friend’s cottage in Northport, on Lake Michigan, I noticed strange rocks on the window sill above the sink. They looked like coral, odd for a cottage on a freshwater lake. My host explained, “Oh, these are Petoskey stones, Michigan’s state rock. Our beach is full of them.” Later that afternoon, while down at the beach, I was stunned to find stone after stone marking the presence of ancient sea life.
Petoskey stones are remnants of once extensive coral reefs. Some 350 million years ago, during the Devonian Age, these reefs edged a large shallow sea covering what is now Michigan. When the sea retreated, the reefs remained. Petoskey stones contain fossils of a tabulate coral, Hexagonaria percarinata, now extinct. When glaciers covered Michigan, moving ice broke apart the fossil reefs, rounding the stones, and delivering them to the beaches of Traverse Bay, where they are found today.
References and more information
Overlay of the Devonian Ocean on the contemporary United States
Further information about the Devonian
Smithsonian, National Museum of Natural History
University of California Museum of Paleontology