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The opening to a small cave at Frontenac State Park looks out onto Lake Pepin. University of Minnesota geologist Greg Brick said these caves hold evidence supporting a 300-year-old claim by French explorer Pierre-Charles Le Sueur that the area has rich saltpeter deposits. (Photo by Greg Brick)

Solving a 300-year-old mystery

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Like any good detective, Greg Brick followed the evidence.

He combed through journals, pieced together maps, collected samples and performed chemical tests.

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After 10 years, Brick, a University of Minnesota geologist from St. Paul, said he found what he was looking for: saltpeter caves in Goodhue County.

“It’s one of the most significant historical finds in the county in many years,” Brick said.

Brick was following the trail of French explorer Pierre-Charles Le Sueur, who traversed the Lake Pepin area some 300 years ago.

In his writings, Le Sueur described finding caves rich in saltpeter, another name for potassium nitrate. They were the earliest known references to the compound — commonly used to make gunpowder — being discovered in North America.

“People had been talking about (the caves) for decades, but no one went out and found them,” Brick said.

According to a translation of Le Sueur’s journal, the explorer discovered the caves along a large lake called “Pein,” which was flanked by mountains to the west and prairies to the east.

Researchers from as far back as the 1960s deduced that Le Sueur was referencing Goodhue County, Brick said, but the location of the saltpeter was never documented.

At first, Brick said his interest in finding the caves was purely historical; but, that changed when his research adviser, Professor Calvin Alexander at the U of M earth sciences department, convinced him to confirm his findings through chemical analysis.

The tests would provide scientific documentation to support his findings, Alexander said.

“It became an interesting project at the intersection of history and geology,” Brick said.

The search started at a recently discovered cave near Sevastopol Bluffs east of downtown Red Wing. Samples taken at the site came back positive for elevated levels of nitrates — a sign Brick was on the right track.

He then shifted his focus to Frontenac State Park, where, with a research permit through the Minnesota Department of Natural Resources, Brick discovered a slew of caves along the park’s northern face.

But the natural landscape at the park proved challenging to collect samples. Steep climbs, loose rocks and the threat of rattlesnakes made for occasionally treacherous outings.

“I had some really close calls,” Brick said.

He said the caves are better described as small crevices, adding that volunteers who agreed to help him in his research were quickly disappointed to find they were nothing like the caves on the Discovery Channel.

“I wore out half a dozen cavers in this project,” Brick said, including local Red Wing cave explorers. He even convinced his fiancée to come along, who went once “and never again,” he joked.

The risk turned out to be worthwhile, though, as soil samples from the caves revealed nitrate concentrations as high as 35,000 parts per million. In comparison, concentrations at historically nitrate-abundant caves in Kentucky rank not much higher at 40,000 parts per million.

The lab work confirmed Le Sueur’s “chemical fingerprints” in Goodhue County, Brick said.

The findings may not prove the caves at Frontenac State Park were the same Le Sueur described in his journal, but it supports the observations made by the explorer in the 1700s, said David Mather, national register archaeologist with the Minnesota Historical Society.

Mather accompanied Brick on one of his sampling expeditions a couple years ago, documenting the experience for the DNR’s Minnesota Conservation Volunteer magazine.

“I thought it was an interesting thing because it really broadened the history of the landscape in the area,” Mather said. “It was an adventure.”

“Historically it was very satisfying,” Alexander said, adding that the research also provided “fundamental insights” into where and how nitrates accumulate in the environment.

After a decade of investigating and a 175-page doctoral dissertation, Brick said he is proud of what he accomplished.

“Yes, and exhausted,” he said.

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Michael Brun
Michael Brun is a graduate of the University of Wisconsin-River Falls journalism program. He has worked for the Republican Eagle since March 2013, covering county government, health and local events. 
(651) 301-7875
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