The Route and Campsites of Lewis and Clark in Montana: A Geologic Perspective

By editor

Dillon, Beaverhead County, Montana

When I consider the vast American West as it unfolded before Lewis and Clark in the early 1800s, I am struck not only by the grandeur of the landscapes but by the subtle, patient record of the earth’s history penned in their journals. The science of geology--now a robust exploration of the earth’s form and substance--was then a fledgling pursuit, little more than a philosophical curiosity. The very term "geology" had only recently entered the lexicon, around 1778, and in the wilds of America, it remained an untamed notion, barely understood, much less practiced.

Meriwether Lewis, setting forth from Camp Dubois in 1804 with Captain William Clark and their company, carried no formal education in the making of mountains, the carving of rivers, or the slow unfolding of sedimentary layers. Yet, what he bore was a keen eye, an unyielding habit of observation, and a journal that would come to hold the earliest field notes of the continent’s geology, although he would hardly have recognized it as such. Theirs was a language of discovery forged in necessity and wonder, employing terms that were as much metaphor as mineralogy, as much borrowed from everyday experience as from classical learning.

In the journals, the term "freestone" appears often--a word that seems to have been Lewis and Clark’s catch-all for any rock that split easily along layers. Today, this would include sandstone and limestone, and sometimes granite, though the latter they also used to describe crystalline rocks in general. They called any hard, silica-rich stone "flint," a word that covered a variety of quartzose rocks, while “marble” referred broadly to any ornamental stone that caught their eye. Their geological vocabulary lacks the precision we now command, but the depth of their observations transcends this limitation.

I have walked much of the route these explorers trod, tracing their campsites along the Missouri River and its tributaries in Montana, places where the land’s ancient story is written in tilted limestone beds and the cliffs of the Missouri Breaks. Lewis marveled at the way the limestone layers tilted skyward, standing firm against the erosive power of the river. To him, these were simply “rocks out of place,” yet his description was detailed and exact enough that modern geologists can trace the angles and strata he noted two centuries ago.

Clark, meanwhile, took measurements of the Jefferson River’s winding course with a precision that astonishes even the modern scientist. His recorded distances and river gradients allow us to reconstruct the river’s profile, understanding how the water’s persistent flow sculpted the valley and shaped the landscape. Their accounts reveal a keen awareness of the relationship between rock and terrain--the way hard limestone ridges soar above the softer shale valleys, how rivers carve channels through yielding rock, leaving the stronger formations to stand sentinel.

This interplay of rock and river, of earth uplifted and water worn, is the narrative of deep time written across Montana’s vast reaches. It is a story that Lewis and Clark, though untrained, began to tell in their journals, a story painstakingly pieced together by generations of geologists since. Robert Bergantino and Kenneth Sandau, of the Montana Bureau of Mines and Geology, have devoted years to correlating the expedition’s descriptions with the region’s actual geology. Their work reveals that the campsites--the very places where the explorers paused to rest and record their observations--can be located with remarkable accuracy by matching the journal’s notes on rock types, river features, and landmarks against known formations.

Standing now on the banks of the Missouri, where the limestone cliffs rise abruptly from the river’s edge, I can almost hear Lewis’s voice in the rustling wind. He wrote, “The rocks here are of a calcareous nature, exhibiting strata inclined at various angles, some almost vertical, and others tilted gently seaward.” Such a passage speaks not only of the rocks themselves but of the profound forces that lifted them from the depths, folding and fracturing them in the restless convulsions of the earth’s crust.

The expedition’s passage through Montana was more than a journey of discovery; it was an unwitting geological survey. Though lacking the technical tools and vocabulary of modern science, the explorers’ detailed, faithful observations form a geological biography of the land. Their records allow us to glimpse the ancient seas that once covered this region, depositing layers of sediment now solidified into limestone and shale, the volcanic activity that laid basalt flows and pumice beds, and the gradual uplift of the Rocky Mountains, which thrust these layers skyward.

The untrained eyes of Lewis and Clark, guided by curiosity and necessity, became the first to document the geological character of this vast, rugged territory. Their journals serve as a window through which we can peer into a time before the geology of the West was understood--a window through which the land’s silent history speaks clearly.

As Clark noted, “The river here winds in many bends, cutting through rock and earth with a force that shapes the land as surely as the hand of time itself.” His words remind me that the landscape is not static but a living record of elemental forces in continuous motion, a record that we, as observers and stewards, must seek to understand with clarity and respect.

To walk the route of Lewis and Clark in Montana is to walk through chapters of earth’s ancient narrative. The campsites they chose were places not only of rest but of observation--points at which the land’s story was briefly held in careful notes and sketches. Thanks to the work of modern geologists who have retraced these steps, matching journal entries to rock formations, we can now map their path with scientific precision, uncovering the deep-time processes that shaped the land and guided the explorers’ way.

In this light, the journey of Lewis and Clark is as much a geological expedition as it was a voyage of political and cultural discovery. Their journals, written in a language of discovery born of necessity, now speak to us across the centuries, telling of tilted strata, river gradients, and rock types with a clarity that belies their early origins. It is a story of the earth’s slow, majestic work--visible in the towering limestone cliffs of the Missouri Breaks, in the meandering waters of the Jefferson River, and in the enduring campsites along their route.

As John Wesley Powell, another great explorer of the West, once remarked, “The geologist studies the earth as a book, and each layer is a line of that book, telling the history of our planet.” Lewis and Clark, in their journals, began reading that book in Montana, turning their observations into the first chapter of a geological story that continues to unfold.

See also

• The Route and Campsites of Lewis and Clark in Montana: A Geologic Perspective at Dillon, Beaverhead County (Montana Bureau of Mines and Geology)
• Camp Fortunate at Dillon, Beaverhead County
• Reliable Landmarks at Dillon, Beaverhead