Why’s That: Petoskey Stones at Van Buren State Park?
Did a local family uncover a new source for Petoskey stones?
Anna VanDeWeert is hunting for rocks on a Lake Michigan beach with her mother, Sandra Widerhold.
“What is it?”
“It looks like a fossil of some sort,” said Anna. “I don't think it's a Petoskey stone, but it still looks pretty cool.”
Last summer at this very spot, Anna was relaxing on the beach when Sandy found about a dozen Petoskey stones just south of a sand dune where a tree had fallen in the water.
This beach is nowhere near the city of Petoskey where you’d expect to find Michigan’s state stone. This beach is more than 250 miles to the south, in Van Buren State Park, near South Haven. In almost 40-years of coming to this spot, Sandy had never made such a find.
“So, the question was, why? I just assumed that this was related to a coral from an inland sea that was much further north,” said Sandy. “So why were they down here in Southern Michigan?”
Petoskey stones are indeed fossilized coral that lived in a shallow sea that covered the state a long time ago, about 400 million years ago to be somewhat exact. Michigan was a very different place when the now extinct coral, called Hexagonaria, thrived in the shallow waters.
“It would have been a nice place for rum and coke. It was a tropical setting located much closer to the equator at that point,” said Peter Voice, a geologist at Western Michigan University.
Voice says Michigan would have looked a lot like the Persian Gulf until plate tectonics moved the state to where it is now. Then relatively recently - that is 10,000 to 15,000 years ago - the glaciers carved out the Great Lakes and just kept on going.
“And that whole mass is then moving south, like a conveyor belt. And so, it's bringing materials from Northern Michigan, or even Canada, into this area that weren't locally derived,” Voice said.
That explains why a Petoskey stone can, theoretically, be found just about anywhere in the state. But a cache of Petoskey stones in one spot so far south, that’s rare.
Rock hunters Sandy, and her daughter Anna, know from experience that the best beaches for Petoskey stones on Lake Michigan are between Charlevoix and Petoskey and along the coast of Lake Huron, from Alpena to Gravel Point.
Sandy wondered if high lake levels in 2019 and 2020 had something to do with the stones they found down here.
“Was it related to Petoskey stones washing down or were they always here and just because the banks were eroding that we found them?”
To find out, we talked to a second geologist named Peter.
Peter Rose is with the Michigan Department of Natural Resources. Together, we went to Sandy and Anna’s lucky spot on the beach to try and solve the mystery.
Rose dismissed the idea that erosion uncovered a new crop of Petoskey stones buried beneath Van Buren State Park.
“That formation isn’t exposed in this area. It’s buried very deeply below where we’re standing right now,” said Rose. “Yeah, they wouldn’t have originated from, from this spot.”
That meant they were transferred here, but how? Rose says lake currents move sediment and rocks around all the time. It’s possible, the fallen tree near the site where Sandy found the rocks trapped them over time. But Rose also has theory that has nothing to do with the lake.
“Finding a Petoskey stone here in general doesn’t surprise me, but a concentration of them, I think is a bit unusual. It’s possible that someone could have brought them here and dropped them or deposited them,” said Rose.
In other words, maybe someone dumped them here.
“That’s not a satisfying answer,” said a disappointed Sandy, whose daughter was okay with it.
“There’s something about somebody dumping them here is kind of amusing,” said Anna, while Sandy mused that perhaps they were left by a vengeful ex.
Sandy and Anna found a bunch of Petoskey stones at Van Buren State Park, but you probably won’t. You will find other cool rocks and you can collect 25-pounds of rocks and minerals on public lands unless restrictions are posted.
Geologist Peter Rose suggested hunting for rocks early in the morning when the water is calm and clear before swimmers kick up the sediment. Or walk along the shore after a storm to see what the waves have washed up. And if you find a Petoskey stone, Rose had a tip for polishing them up.
“If you don’t have a rock polisher and you want to semi-polish Petoskey stone, you just carry it around in your pocket for long periods of time and the lint in your pocket will smooth it and polish it up a bit,” Rose said.