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EGLE rockhound: Keep an eye out for rocks dragged hundreds – and even thousands – of miles by ancient glaciers into Michigan

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.

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.

A Gowganda tillite rock and a unique glacial erratic rock, both transported by a glacier and deposited in a location different from their origins.
Unique glacial erratic rock up close in hand

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.