56 Minnesota Conservation Volunteer On theTrail of Le Sueur At a state park on the Mississippi River, a geologist

investigates a historical mystery—a 17th-century

search for saltpeter.

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 By David Mather 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 Way up Rattlesnake Bluff, geologist bears in winter and rattlesnakes in summer. Greg Brick examines the entrance to a He said all of them contained saltpeter—either cave, one of dozens he’s discovered in

DAVID MATHER MATHER DAVID and around Frontenac State Park. 57 through a zone of huge, moss-covered boulders of dolomite “float”—sofa­ and bus-sized chunks of bedrock that The state park’s rock crevices and broke away and tumbled down from bluff slopes create an array of the cliff long ago. Sidestepping large, microenvironments, supporting plants white flowers of trillium and clusters of such as large-flowered trillium. wild ginger, we ascend. As we approach the vertical cliff of exposed Oneota potassium nitrate or a naturally occur­ dolomite, I begin using my hands to aid ring precursor such as calcium nitrate. the climb. A large, black shape passes According to Brick, Le Sueur made close behind me, and I realize we’ve the earliest known reference to cave reached the level of a circling kettle of saltpeter in North America. It is a turkey vultures. compelling example of early geological Soon, we’re standing on a narrow prospecting for valuable minerals. shelf at the base of the dolomite. At this PHOTOGRAPHS BY DAVID MATHER MATHER BY DAVID PHOTOGRAPHS Saltpeter is a component needed to level, the crags and openings of the cliff make gunpowder. Le Sueur might have create eerie, phantomlike shapes. One found an important resource for the outcrop appears to have the leering French in the wilderness of the upper eyes of a skull. Mississippi River. Nearby, as we look, the cave entrance But did the French actually mine the suddenly appears. Brick climbs up Lake Pepin caves for saltpeter? Or per­ inside, and I join him. Standing in the haps Le Sueur was too optimistic in his lower level of a small cavern with two description—as he had been with his tiers, we see the floor is littered with discovery of the Minnesota River “cop­ long, striped feathers from the roost per mine” that produced only blue clay. of a large bird—probably a raptor, we Intrigued by the cryptic historical guess, because of the chomped bones references to saltpeter caves, Brick of small animals we’d spotted around obtained a research permit from the the entrance. Tan, sugary sparkles of Department of Natural Resources “cinnamon” quartz glitter faintly in the and began exploring the state park’s rough dolomite walls. backcountry for clues. Brick hopes Brick gathers soil samples from both to determine the likelihood that the levels. On the upper tier, the ceiling is so French mined nitrates for saltpeter. low that he has to crawl amid raccoon His discovery could add another scat. He also collects soil outside the dimension to our understanding of cave for a control sample. this pivotal time in history. Descending the bluff, Brick tells me that Le Sueur was getting ahead of Terra Rossa. It’s the middle of May, himself by saying he’d found saltpeter. the peak of the spring warbler migration. Potassium nitrate can occur naturally, Our climb up Rattlesnake Bluff begins but in caves it is more common to find

58 Minnesota Conservation Volunteer 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 salt­ peter. 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 ex­ plains. Sheltered from rainwater, the nitrates left by decomposing plants and animals accumulate simply because they do not wash away. Nitrate accu­ mulation 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 accumu­ late. He half-jokes that if he sees sta­ lagmites 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 MATHER BY DAVID PHOTOGRAPHS trip. They are above Point-No-Point, a The Oneota dolomite of Rattlesnake Bluff also contains stretch of shoreline historically named Prairie du Chien chert (top), a harder rock. Greg Brick for an optical illusion of Lake Pepin examines the tight space of a cave formed by shifting travelers who mistook this gradual slabs of rock.

January–February 2013 59 PHOTOGRAPHS BY DAVID MATHER Weathering and the pull of gravity on the Lake Pepin bluffs create crevices, caves, and many small openings, including these that give an illusion of a spooky face. Geologist Greg Brick has dubbed this formation the “Old Man of Frontenac.”

60 Minnesota Conservation Volunteer This state park along the Mississippi River offers an ideal setting for camping, hiking, and exploring nature. The hiking trails on Point-No-Point have stairways and switchbacks that afford visitors spectacular views of Lake Pepin and close-up looks at the geology and the plant life of bluff slopes.

January–February 2013 61 curve for a point of land in the near dis­ tance—an illusory point that can never be reached. Playing off the name, Brick calls our destination “Cave No Cave,” because he sometimes has trouble find­ ing 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 DAVID MATHER DAVID 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 eye­ brow, he tells me that he was last at this cave in 2004 and it was full of cave crick­ ets. 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 fol­ low, twisting my torso upward through the short entrance passage. DAVID MATHER MATHER DAVID The cave—technically a “mechani­ cal 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

ALLEN BLAKE SHELDON rocks. With the exception of a solitary Greg Brick has found that even small crevices in the park silverfish insect, we see no inhabitants. can contain concentrations of nitrates (above). The Brick scribbles notes on a data sheet and bluff trail offers visitors the best view of the natural scrapes soil from the floor into a sample beauty of the In-Yan-Teopa stone arch. bag for laboratory analysis.

62 Minnesota Conservation Volunteer Brick points out that without find­ ing artifacts, it’s impossible to know which specific locations the French vis­ ited. And while the Rattlesnake Bluff The ecological mosaic of Frontenac State Park cave—formed in an ancient karst land- makes it a destination for birders in search of scape—would have been there in Le migrating warblers, such as the Cape May war­ Sueur’s time, Brick has come to suspect bler (lower left), yellow-rumped warbler (upper that some of these mechanical-crevice right), and Wilson’s warbler (middle right). ALLEN BLAKE SHELDON caves could be younger. Gravity keeps pulling at the bluff edges, erasing old ing a long-lost friend. A light, misting crevices and creating new ones where rain floats on the breeze. I follow two nitrates accumulate. The timing of that white-tailed deer, and I pause to watch process is unclear, but the cave soil as a towboat churns the water, pushing samples Brick has tested so far contain four barges downstream. Tree swal­ nitrates sufficient to make saltpeter. lows skim the choppy water. An American redstart catches my eye Classic Arch. After a day of hard as it lands near my shoulder. Peering climbing, we say goodbye at the top of through the mist, I suddenly see red- ALLEN BLAKE SHELDON the bluff. Brick is heading home, and I starts and other warblers everywhere, return to my campsite in the park. flitting over rocks and driftwood and The next morning I linger over cof­ from branch to branch. Within min­ fee at camp, thinking about the changes utes the flock is gone, as quickly as it to the world in the centuries since Le had appeared. Sueur’s exploration. I decide to explore I came to Frontenac State Park fo­ the bluffs further, and I follow the park cused on caves. Since he started this trail to a graceful, dolomite arch. Known research in 2004, Brick has found doz­ by its Dakota name, In-Yan-Teopa, the ens of previously unknown caves in arch is a classic example of gravity’s work and around the park. His analysis of on the bluff edges: It formed from the the soils shows that Le Sueur was right slow, slow, downward crumbling of the about the presence of saltpeter (or bedrock over thousands of years. more accurately, about the concentra­ Descending on the trail switchbacks, tion of nitrates). These discoveries add I see that the arch is best viewed from to the park’s history and complement below. I feel the pull of gravity and note its many geological wonders. its effects all around me. Though still The park’s geology is the foundation MATHER DAVID solid, the trail’s wood-and-stone steps of an ecosystem and a stage for history. have been slowly pulled and twisted. I’ve seen Brick traverse the sides of these The slope is littered with fallen trees bluffs like a billy goat. By accompany­ and boulders. ing him, I’ve learned that Le Sueur must Reaching the comfort of level have done that too—in fact, right now I ground at the river feels like meet- can actually feel it in my bones. nV

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