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EGLE rockhound: Keep an eye out for rocks dragged hundreds – and even thousands – of miles by ancient glaciers into Michigan
May 22, 2025
Today’s MI Environment story, by Kent Walters, a geologist in the Materials Management Division of the Michigan Department of Environment, Great Lakes, and Energy (EGLE), is the latest in a series of stories on rock and mineral identification.
Rocks in fields, rocks on the beach, rocks in the hole you’re trying to dig, but where do all these rocks come from in Michigan? The answer to this question begins thousands of years ago during the last glacial maximum in Michigan.
Approximately 30,000 years ago temperatures started to cool which allowed significant growth of large icesheets worldwide. One of these ice sheets known as the Laurentide Ice Sheet grew rapidly, migrating through Canada, and into the United States.
The Laurentide Ice sheet was so large, and so powerful that it broke off, pulverized and carried along rocks of all sizes and varieties as it grinded over the Canadian landscape. At one point, the entire State of Michigan was covered by ice. When temperatures started to warm again, and the ice retreated back north, it left behind all the hitchhikers that it picked up while traveling through Canada. Geologist call rocks that have been transported by glaciers glacial erratics. Glacial erratics vary in size with some that can be larger than your car!
Pictured: EGLE geologist Kent Walters atop a glacial erratic rock in the Upper Peninsula.
As a trained glacial geologist and a rockhound on the weekends, I grew an appreciation for the Laurentide Ice Sheet and all the glacial erratics it left behind.
It’s almost certain that if you pick up one rock, the one next to it will be completely different and unique. Many rockhounds like to travel to specific places to look for specific rocks or minerals, but I would argue that some of the most interesting and unique rocks can be found at your local beach or farm field right here in Michigan. In Michigan, the most commonly sought-after glacial erratics by rockhounds include puddingstones, lightning stones, Yooperlites®, or Lake Superior agates.
Puddingstone is a general description of a quartzite conglomerate with bright red jasper inclusions. Lightning stones were formerly known as septarian nodules, and Yooperlites® are minerals called sodalite that fluoresces under a black light. Beyond these most sought-after glacial erratics, there are plenty more unique, interesting and beautiful rocks to find.
Pictured: A Gowganda tillite rock and a unique glacial erratic rock, both transported by a glacier and deposited in a location different from their origins.
So where is the best place to hunt for rocks? Like morel mushroom hunters, many rockhounds keep those locations a secret, but we can look to glacial geology to give us one clue for where a good rock hunting location might be. During the retreat of the Laurentide Ice Sheet it sometimes left behind distinct features called glacial moraines. These are features that typically marked the position of the Laurentide Ice Sheet where it piled up a range of unconsolidated soil types and glacial erratics. While it’s not difficult to locate glacial moraine features with the available surficial geology maps, the hard part is finding a location to easily access the rocks. Finding locations where glacial moraines intersect a Great Lake, however, allows the constant wave action to do the hard work for you. Decades and decades of constant erosion of these glacial moraine bluffs allows the wave action to erode out the many glacial erratics that are contained within. While the soil is washed away, a beach full of rocks can be left behind.
Glacial moraines are just one of many tips that can be used to find a good rock hunting location, and there are plenty of other rock collecting locations that are not related to glacial moraines. Sometimes the annual plowing of a farm field can be just as fruitful. Once you start paying attention, glacial erratic rocks start popping up everywhere.
Rockhounding is fun but be mindful to seek permission to collect rocks on private properties and know that collecting rocks on public properties is limited by the State of Michigan to 25 pounds per person annually.