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Image of Frontenac State Park.

On the Trail of Le Sueur

At a state park on the Mississippi River, a geologist investigates
a historical mystery—a 17th-century search for saltpeter.

by David Mather

Rattlesnake Bluff towers above Lake Pepin, a 21-mile-long widening of the Mississippi River, southeast of Red Wing. Wooded, rock-strewn slopes lead sharply up—way up—to a thick band of pale gray, vertical bedrock.

Here at the foot of the bluff, University of Minnesota geologist Greg Brick hands me a blue helmet, then straps a bright red one over his fiery red hair. He is searching Frontenac State Park for small caves, as French explorer Pierre-Charles Le Sueur did more than 300 years ago. One of the caves is straight up there.

Lake Pepin was among the farthest frontiers of New France in the 1690s, when Le Sueur operated a trading post nearby among Dakota Indians. When he returned in the year 1700, he continued to explore the region's riverside caves. Le Sueur described the caves as the haunts of bears in winter and rattlesnakes in summer. He said all of them contained saltpeter—either potassium nitrate or a naturally occurring precursor such as calcium nitrate.

According to Brick, Le Sueur made the earliest known reference to cave saltpeter in North America. It is a compelling example of early geological prospecting for valuable minerals. Saltpeter is a component needed to make gunpowder. Le Sueur might have found an important resource for the French in the wilderness of the upper Mississippi River.

But did the French actually mine the Lake Pepin caves for saltpeter? Or perhaps Le Sueur was too optimistic in his description—as he had been with his discovery of the Minnesota River "copper mine" that produced only blue clay.

Intrigued by the cryptic historical references to saltpeter caves, Brick obtained a research permit from the Department of Natural Resources and began exploring the state park's backcountry for clues. Brick hopes to determine the likelihood that the French mined nitrates for saltpeter. His discovery could add another dimension to our understanding of this pivotal time in history.

Terra Rossa. It's the middle of May, the peak of the spring warbler migration. Our climb up Rattlesnake Bluff begins through a zone of huge, moss-covered boulders of dolomite "float"—sofa- and bus-sized chunks of bedrock that broke away and tumbled down from the cliff long ago. Sidestepping large, white flowers of trillium and clusters of wild ginger, we ascend. As we approach the vertical cliff of exposed Oneota dolomite, I begin using my hands to aid the climb. A large, black shape passes close behind me, and I realize we've reached the level of a circling kettle of turkey vultures.

Soon, we're standing on a narrow shelf at the base of the dolomite. At this level, the crags and openings of the cliff create eerie, phantomlike shapes. One outcrop appears to have the leering eyes of a skull.

Nearby, as we look, the cave entrance suddenly appears. Brick climbs up inside, and I join him. Standing in the lower level of a small cavern with two tiers, we see the floor is littered with long, striped feathers from the roost of a large bird—probably a raptor, we guess, because of the chomped bones of small animals we'd spotted around the entrance. Tan, sugary sparkles of "cinnamon" quartz glitter faintly in the rough dolomite walls.

Brick gathers soil samples from both levels. On the upper tier, the ceiling is so low that he has to crawl amid raccoon scat. He also collects soil outside the cave for a control sample.

Descending the bluff, Brick tells me that Le Sueur was getting ahead of himself by saying he'd found saltpeter. Potassium nitrate can occur naturally, but in caves it is more common to find weaker nitrate concentrations that must be mined and processed to produce saltpeter. Brick knows that the cave floor's reddish soil, called terra rossa (red earth), is rich in nitrates. In his lab at the university, Brick will measure the concentration of nitrates in the samples. He will compare the nitrate content with that of soils in caves historically known for nitrates mined to manufacture saltpeter. If the levels are comparable, it will show that saltpeter could be made from the Lake Pepin caves.

Nitrates accumulate in dry caves through natural processes, Brick explains. Sheltered from rainwater, the nitrates left by decomposing plants and animals accumulate simply because they do not wash away. Nitrate accumulation is enhanced by a potent brew of feces, urine, debris from raptor nests, and other organic material.

These caves were noteworthy to Le Sueur because they were dry, Brick says. The karst landscape of southeastern Minnesota has lots of caves, but most are solution caves, formed by underground streams. Water movement there washes nitrates away before they can accumulate. He half-jokes that if he sees stalagmites or stalactites when he enters a cave, he usually just leaves because those formations indicate solution caves.

Cave No Cave. We travel to a steep bluff below the picnic area to visit other caves that Brick had found on a previous trip. They are above Point-No-Point, a stretch of shoreline historically named for an optical illusion experienced by Lake Pepin travelers who mistook this gradual curve for a point of land in the near distance—a point that can never be reached. Playing off the name, Brick calls our destination "Cave No Cave," because he sometimes has trouble finding one of them again.

Brick scans our surroundings as we traverse the bluffside, our ankles and knees bending at unnatural angles to accommodate the steep slope. Then he stops below a large knob of cream-colored rock and starts pulling gear out of his backpack. I spot a small hole on the underside of the outcrop.

Brick quickly reads through the notes on his metal clipboard. Raising an eyebrow, he tells me that he was last at this cave in 2004 and it was full of crickets. After repacking the clipboard, he lies down on the slope, sticks his head and arms into the hole, and disappears. My feet scramble for purchase as I follow, twisting my torso upward through the short entrance passage.

The cave—technically a "mechanical crevice," Brick says—is narrow, just slightly broader than my shoulders and not quite tall enough for us to stand up straight. The walls of this tight enclosure were formed when gravity pulled a slab of the bedrock slightly away from the bluff. Over time, other rocks tumbled down and formed a ceiling by getting wedged in the crack.

Pale greenish daylight seeps through the gaps between mossy, fern-covered rocks. With the exception of a solitary silverfish insect, we see no inhabitants. Brick scribbles notes on a data sheet and scrapes soil from the floor into a sample bag for laboratory analysis.

Brick points out that without finding artifacts, it's impossible to know which specific locations the French visited. And while the Rattlesnake Bluff cave—formed in an ancient karst landscape—would have been there in Le Sueur's time, Brick has come to suspect that some of these mechanical-crevice caves could be younger. Gravity keeps pulling at the bluff edges, erasing old crevices and creating new ones where nitrates accumulate. The timing of that process is unclear, but the cave soil samples Brick has tested so far contain nitrates sufficient to make saltpeter.

Classic Arch. After a day of hard climbing, we say goodbye at the top of the bluff. Brick is heading home, and I return to my campsite in the park.

The next morning I linger over coffee at camp, thinking about the changes to the world in the centuries since Le Sueur's exploration. I decide to explore the bluffs further, and I follow the park trail to a graceful, dolomite arch. Known by its Dakota name, In-Yan-Teopa, the arch is a classic example of gravity's work on the bluff edges: It formed from the slow, slow, downward crumbling of the bedrock over thousands of years.

Descending on the trail switchbacks, I see that the arch is best viewed from below. I feel the pull of gravity and note its effects all around me. Though still solid, the trail's wood-and-stone steps have been slowly pulled and twisted. The slope is littered with fallen trees and boulders.

Reaching the comfort of level ground at the river feels like meeting a long-lost friend. A light, misting rain floats on the breeze. I follow two white-tailed deer, and I pause to watch as a towboat churns the water, pushing four barges downstream. Tree swallows skim the choppy water.

An American redstart catches my eye as it lands near my shoulder. Peering through the mist, I suddenly see redstarts and other warblers everywhere, flitting over rocks and driftwood and from branch to branch. Within minutes the flock is gone, as quickly as it had appeared.

I came to Frontenac State Park focused on caves. Since he started this research in 2004, Brick has found dozens of previously unknown caves in and around the park. His analysis of the soils shows that Le Sueur was right about the presence of saltpeter (or more accurately, about the concentration of nitrates). These discoveries add to the park's history and complement its many geological wonders.

The park's geology is the foundation of an ecosystem and a stage for history. I've seen Brick traverse the sides of these bluffs like a billy goat. By accompanying him, I've learned that Le Sueur must have done that too—in fact, right now I can actually feel it in my bones.

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