John Jengo is a professional geologist and licensed Site Remediation Professional who works for an environmental consulting firm in Pennsylvania, specializing in hydrocarbon remediation and dam removals to restore migratory fish passage. He has published numerous articles in We Proceed On since 2002 on the subject of Lewis and Clark’s mineral collection and the significance of scientific influence of their geological discoveries.
Contributions
Phoca (Seal) Rock
by John W. Jengo
The mid-river island identified as “Phoca” and “Seal rock” on one of William Clark’s route maps is a compact landslide block that detached from the Cape Horn headland.
The Missouri River exposed rock formations that were geologically diverse, distinctly colored, rich in mineral content, and in some places, dramatically distinguished by steaming and smoking hot earth that beckoned to be investigated.
Searching for Lead
by John W. Jengo
In the afternoon of 4 June 1804, William Clark decided to investigate the purported occurrence of lead in the vicinity of a rather unique prominence he named “Mine Hill,” but which is known today as Sugar Loaf Rock. The search was unsuccessful, but Lewis’s previous inquiries while in St. Louis resulted in 11 specimens sent to Thomas Jefferson.
From the moment they dipped their paddles into the Snake River, the Lewis and Clark expedition would not only be following the path of multiple flood basalt flows, but also the route of the colossal Lake Missoula floods that so markedly scoured the basaltic landscape.
The 21 October 1805 rock fall geologic notation is likely referring to mass-wasting features along a fourteen-mile long stretch of dramatically steep slopes visible today between Blalock Canyon and the John Day River.
Dubbed “Ship Rock” on Clark’s route map, but now known as Monumental Rock, this distinct outcropping of the Lower Monumental Member of the Saddle Mountains Basalt is located on the south bank of the Snake River in Walla Walla County, Washington, just upriver of Lower Monumental Dam.
Geology of The Dalles
by John W. Jengo
Over four days (22-25 October 1805) the expedition would be navigating through one of the most exciting and difficult stretches of the entire water route to the Pacific. Several outstanding geological features were noted.
A Submerged Forest
by John W. Jengo
On 30 October 1805, Clark documented the presence of a submerged forest, which along with the burning bluffs of northeastern Nebraska, the “Burnt Hills” of North Dakota, and White Cliffs of the Missouri in central Montana, remain one of the expedition’s most famous geological observations.
As a professional geologist and a Lewis and Clark enthusiast I’m impressed by what the captains had to say about the geology of the Upper Missouri River Breaks, as suggested by the following journal excerpts and commentary.
Rooster Rock at Crown Point
by John W. Jengo
The promontory of Crown Point comes to mind when Clark mentions in his notebook journal that the expedition camped “under a high projecting rock.”
Although not nearly as celebrated as their botanical and zoological work, Lewis and Clark collected a multitude of mineralogical specimens throughout the expedition.
Near Wallula Gap, Meriwether Lewis apparently collected an enigmatic and long-missing mineral specimen that was never documented in the expedition journals.
We may never know the full historical impact of Lewis and Clark’s discoveries upon nineteenth-century scientific inquiry, but this example highlights how just a series of conversations with the returning explorers allowed a significant earth science discovery to be revealed to the scientific community.
Columbia River Basalts
by John W. Jengo
The region of the lower Snake River and the Columbia River’s course through the Columbia Plateau and Gorge experienced volcanic activity starting some 55 million years ago, although the expedition would primarily encounter rock units of Miocene-age.
Columbia Gorge Waterfalls
by John W. Jengo
William Clark: “Saw 4 Cascades caused by Small Streams falling from the mountains on the Lard. Side.”
The expedition traveled out from under an ancient river channel frozen in time to a river discharging huge volumes of sediment in real time, the “quicksand river,” now known as the Sandy River.
The consolidated rocks that compose the Gorge were formed by a complex interplay of Columbia River Basalt Group flood basalt deposition and basin subsidence, along with contemporaneous folding and faulting—not the Glacial Lake Missoula floods.
Experience the Lewis and Clark Trail
The Lewis and Clark Trail Experience—our sister site at lewisandclark.travel—connects the world to people and places on the Lewis and Clark Trail.
Discover More
- The Lewis and Clark Expedition: Day by Day by Gary E. Moulton (University of Nebraska Press, 2018). The story in prose, 14 May 1804–23 September 1806.
- The Lewis and Clark Journals: An American Epic of Discovery (abridged) by Gary E. Moulton (University of Nebraska Press, 2003). Selected journal excerpts, 14 May 1804–23 September 1806.
- The Lewis and Clark Journals. by Gary E. Moulton (University of Nebraska Press, 1983–2001). The complete story in 13 volumes.
This site is provided as a public service by the
Lewis and Clark Trail Alliance
with cooperation and funding from the following organizations:
- Lewis and Clark National Historic Trail
- Montana Committee for the Humanities
- Idaho Governor's Lewis and Clark Trail Committee
- The Capital Gallery (Lewis and Clark Fort Mandan Foundation)
- Yellowstone Public Radio
Unless otherwise noted, journal excerpts are from The Journals of the Lewis and Clark Expedition, edited by Gary E. Moulton, 13 vols. (Lincoln: University of Nebraska Press, 1983-2001).