Geologists uncover new clues about how glaciers shaped the St. Croix Valley

Recent research near Osceola has answered old questions about Ice Age events.

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5 minute read

Travel north, east, or south from Osceola, Wisconsin, and you’ll cross a fairly flat plain dotted with farms and housing developments. Then, within a few miles, you will end up climbing a large hill. These slopes are the banks of an ancient river that once raged with water pouring out of a vast glacial lake. The prehistoric torrent affected not just the area’s topography but also subsequent history, land use, and human activity.

Geologists have recently learned more about how this land formation was created, and improved their overall understanding of the Ice Age events that formed the St. Croix River and its valley. As part of a broad project studying how glaciers shaped landscapes, funded by the National Science Foundation, Hunter Delikowski, a graduate student at Minnesota State University, Mankato, and collaborators are studying the St. Croix Valley’s specific role in the story.

“There is a lot of complexity to the St. Croix’s glacial history,” Delikowski says. “It’s just amazing, all the stuff that’s happened to this one river valley. It’s spectacular.”

Field study photos courtesy Hunter Delikowski.

Delikowski and his colleagues have conducted intensive field research around the St. Croix Valley, including at Standing Cedars Community Land Conservancy and William O’Brien State Park, taking soil samples and using ground-penetrating radar to study the subsurface stories. Back in their labs, they have performed extensive analysis, and found evidence of large floods which once flowed through what’s now eastern Polk County. The work has confirmed some earlier suspicions and revealed a lot of new details about prehistoric geologic events.

Today, the St. Croix begins just thirty miles south of Lake Superior, which drains east through the other Great Lakes, and eventually the Atlantic Ocean, while the St. Croix flows into the Mississippi and ultimately the Gulf of Mexico. But the lake once rose 500 feet above its current level, spilled over a ridge, and carved the St. Croix Valley, for a while draining the Greatest Lake toward the Mississippi.

That series of events has been fairly well understood for more than a century. But the story of the St. Croix Valley is not so simple. The land has repeatedly been flattened by glaciers, covered by broad lakes, and carved by meltwater.

“The magnitude, duration, frequency, and chronology of the meltwater flooding events from these lakes into the St. Croix are not well known,” Delikowski says. “Therefore, their impact on the river is not well known.”

As the story of the St. Croix Valley’s creation continues to unfold, Delikowski’s team is building on research from the 1870s to the 2000s, contributing to a span of scientific study of the St. Croix River and its valley that stretches back more than 150 years.

The old meltwater valley by Osceola was identified and named in a 2000 report on Polk County’s geology during the Ice Age by Mark Johnson of the Wisconsin Geological and Natural History. Johnson dubbed the formation the “Osceola Bench,” noticing surface deposits ranging from sand to boulders, and surmised it had been created by glacial meltwater.

“He made this using topographic maps, I think at 20-foot contours,” Delikowski says. “It’s just incredibly impressive that he was able to make this in 2000.”

Two decades later, Delikowski is testing the theories, and Johnson is one of the collaborators on the research.

Field work photos courtesy Hunter Delikowski.

One of the complicating factors of Ice Age geology is that glaciers did not simply expand south in a steady pace and direction, and then return north in a similar fashion. Instead, they advanced and retreated, sent lobes on side quests, melted and froze. There were times when one part of a glacier would be headed south while another part returned north. Meltwater lakes and rivers came and went.

“The chronology of this is very poorly understood,” Delikowski says.

Because of all this, there was no single event that created the St. Croix River and its valley as it is known today. The river itself has evolved over the past ten or twenty millennia, its course wandering across the landscape, its water constantly cutting down into soft rock. It is alive today, still slowly but steadily carving its course, change its only constant.

This particular chapter in geologic history is a prime example of the irregularities of ice sheets. About 16,000 years ago, a lobe of ice that was pushing south through Minnesota sent out an offshoot that advanced northeast, ultimately stopping near Grantsburg, Wis. Here, where a glacier had come from the south, the ice dam trapped water melting off to the north.

“It’s kind of overwhelming, the sheer amount of things that are affecting this river in such a short amount of time,” Delikowski says. “It’s part of what’s so beautiful and fascinating about the river.”

As the ice pressed and shifted against the nascent St. Croix Valley, water started to leak south. As it carved down through the bedrock, the flow grew. As the ice retreated, water so powerful it carried boulders crashed through and sculpted the flat valley where the villages of Osceola and Dresser sit today. Eventually, the water slowed and slackened, and carried only sand and silt. And then, it stopped flowing altogether.

The different types of sediment found in different parts of the St. Croix Valley speak to these varying events. Drilling down into the ground, below the topsoil, one might encounter the silt or clay of an old lake bottom, the sand of a slow-moving stream, the cobble of a large creek, or the boulders of a glacial meltwater river.

A trap rock quarry has operated in Dresser for more than a century. It happens to be located where Delikowski theorizes there may once have been a mighty waterfall at the outlet of Glacial Lake Granstburg. There is a good chance it’s an ideal place to mine billion-year-old basalt because other material on top had been washed away by the meltwater.

“We think that this spillway is the outlet of Glacial Lake Grantsburg,” Delikowski says. “There’s so little sediment here that it makes this a perfect area for them to mine out that trap rock.”

The Osceola Bench, the outlet of Glacial Lake Grantsburg, is only a few miles from the St. Croix Dalles, which was also carved by glacial melt. When waters came rushing south from Lake Superior and hit the exposed basalt created by long-ago lava flows, it smashed open a passage through the rock and swirled stones so powerfully that it drilled deep “potholes” in the rock, which are now a popular attraction at Interstate Park.

Based on Delikowski’s research and other geologic evidence, it now appears the Dalles were formed a couple thousand years after meltwater from Glacial Lake Grantsburg and other events started carving the lower St. Croix Valley. In fact, he has also found evidence how the later flows carved into the pre-existing formations of the Osceola Bench.

Delikowski is currently wrapping up his thesis, when he will formally present his full findings. As he does so, more answers will emerge about the complicated glacial events that created the St. Croix Valley as it is known today.


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5 responses to “Geologists uncover new clues about how glaciers shaped the St. Croix Valley”

  1. Mark Hove Avatar
    Mark Hove

    Cool!

  2. Jeff Rivard Avatar
    Jeff Rivard

    This is extremely good work. I hope he will do a presentation of it someday. We drive by these geological features every day, without thinking about how they were formed.

  3. Jeff Willius Avatar

    Fascinating story! I’ll stay tuned for further discoveries.

  4. Matthew Mraz Avatar
    Matthew Mraz

    I am eager to learn about Delikowski’s findings. These results will serve as a foundation for future researchers to build upon.

  5. KateC Avatar
    KateC

    Wow! I’ve long known that massive amounts of glacial meltwater created the Dalles at Taylor’s Falls, but fascinating to learn of the other ancient river channels and an explanation of “the bench.” Thanks

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