Evidence of the Expedition: Clark on the Yellowstone

By editor

Glendive, Dawson County, Montana

On a summer day in 1806, Captain William Clark, one half of the Corps of Discovery, found himself confronting the vast and uncharted expanse of the northern Yellowstone River valley. On July 25, near what would one day be called Pompeys Pillar, Clark stopped to inscribe his name on a sandstone outcrop--a record that would outlast many of the expedition’s more ephemeral traces. His journal entry reads with deliberate clarity: "4 P M arived at a remarkable rock. …This rock which I shall Call Pompys Tower. …I marked my name and the day of the month & year." This simple act of carving into stone, performed with a knife on the soft sandstone face that rises nearly 150 feet above the Yellowstone, endures as the sole on-site physical evidence of the entire 8,000-mile journey undertaken by Lewis and Clark’s Corps of Discovery.

The rock itself is a geological marvel. Pompeys Pillar is composed of sandstone from the Hell Creek Formation, a sedimentary layer deposited approximately 66 million years ago during the late Cretaceous period. This same formation contains fossils of the last non-avian dinosaurs, yet the pillar towers above the river as a monument to a far more recent chapter of natural and human history. The pillar’s vertical face is weathered by the relentless cycles of freeze and thaw typical of this semi-arid region, but the inscription remains remarkably intact, shielded from erosion by the overhanging ledges and the dry air.

Clark and his party had been traveling along the north side of the Yellowstone River since early July, after departing Travelers’ Rest, the winter camp near present-day Lolo Pass. Their progress was slowed by the loss of horses, the expedition’s primary mode of transport through the rugged terrain. On July 24, Clark recorded a grievous setback: twenty-four horses vanished, likely taken by Crow raiders. Horses were vital not only for carrying men and equipment but also for sustenance and trade. In response, Clark decided to abandon overland travel in favor of the river route, ordering the construction of two canoes made from cottonwood logs felled along the riverbank. These canoes were lashed together to provide stability for the journey downstream.

Clark’s decision to shift from horse to watercraft reflects a keen understanding of the landscape’s physical demands. The Yellowstone River here cuts through sedimentary plains and rolling hills, with intermittent patches of cottonwood (Populus deltoides) and willow (Salix spp.) along the banks. The river’s flow is steady but can be swift, and navigating it required skill and caution. Clark’s journal notes from July 20 reveal his anticipation: "I deturmined to have two Canoes made out of the largest of those trees and lash them together which will Cause them to be Study and fully Sufficient to take my Small party & Self with what little baggage we have down this river."

Meanwhile, Sergeant Nathaniel Pryor and three privates--Hall, Shannon, and Windsor--were dispatched with the remaining horses to find a more secure route. Unfortunately, these horses were also stolen. The four men, stripped of their mounts, trekked overland to the Yellowstone River near Pompeys Pillar, where they constructed bull boats. These were circular rafts made by stretching buffalo (Bison bison) hides over a frame of willow branches, laced tightly with rawhide thongs. Bull boats were lightweight and buoyant--ideal for floating downriver but vulnerable to rough currents. The men floated down the Yellowstone and Missouri rivers, reuniting with Clark’s party on August 8. Eventually, the entire group joined Captain Meriwether Lewis’s contingent on August 12 near present-day Williston, North Dakota.

The loss of horses and the transition to river travel highlight the precarious balance the expedition maintained between the land’s natural obstacles and the presence of native peoples. Clark’s journal entry on July 23 reads: "Sgt. pryor found an Indian Mockerson and a Small piece of a roab … those Indian Signs is Conclusive with me that they have taken the 24 horses which we lost." The Crow--known to the expedition as the Absaroka--were established inhabitants of this region, skilled horsemen and traders whose control of the northern Yellowstone valley complicated the Corps’ passage.

As the canoes drifted downstream past the mouth of the Bighorn River on July 27, Clark wrote with a mixture of melancholy and finality: "when we pass the Big horn I take my leave of the view of tremendious chain of Rocky mountains white with Snow." The Rockies, snowcapped and rugged, receded from view as the expedition moved into the rolling plains. This moment marked the last sighting of the mountains for Clark, a poignant farewell to the towering geological formations that had defined much of the expedition’s western journey.

The landscape around Pompeys Pillar also holds botanical interest. The cottonwoods along the Yellowstone floodplain provide essential shelter and food sources for local fauna, including mule deer (Odocoileus hemionus), beaver (Castor canadensis), and a variety of migratory birds. The dry grasslands here support blue grama (Bouteloua gracilis) and needle-and-thread grass (Hesperostipa comata), species adapted to the semi-arid climate marked by hot summers and cold winters. The pillar’s position near the river’s edge makes it a prominent landmark for both human and animal travelers.

Clark’s inscription itself has drawn the attention of historians and naturalists alike. It is one of the few direct, tactile connections to the Corps of Discovery’s epic 1804-1806 voyage. As historian Stephen Ambrose noted, Clark’s signature "is the only physical evidence of the Lewis and Clark Expedition along the entire route that remains where it was left." The expedition left no permanent settlements, no structures, only traces in journals, sketches, and this solitary carving.

In reflecting on this, Clark’s words resonate with a sense of purpose and observation that matches the naturalist’s spirit. The expedition was a scientific and diplomatic mission, aiming to chart unknown lands, document flora and fauna, and establish relations with indigenous nations. Clark’s recording of natural landmarks, and his choice to leave his mark on Pompeys Pillar, capture an intersection of human curiosity and the vastness of the American West.

Pompeys Pillar remains accessible today along the Yellowstone River near Glendive, Montana, preserved as a National Monument since 2001. Visitors can stand beneath the sandstone face and see, etched in the rock, Clark’s unmistakable cursive signature. The pillar continues to invite reflection on the interplay of geology, history, and the endurance of human endeavor.

See also

• Evidence of the Expedition at Glendive, Dawson County
• Clark's Canoe Camp on the Yellowstone at Park City, Stillwater County
• Captain Clark and the Corps of Discovery on the Yellowstone at Livingston