Lewis and Clark in Montana — a geologic perspective

Powder River and the Terry Badlands

Location map

As Clark descended the Yellowstone River he dutifully noted the many natural features he saw. A modern traveler here still can see and appreciate what he described:

Friday July 30th, 1806 At the mouth of the Powder River, Clark noted:

. . . the water . . . is 100 yds wide, the bead to this river nearly ¼ of a mile this river is Shallow and the water very muddy and of the Colour of the banks a darkish brown. I observe great quantities of red Stone thrown out of this river that [and] from the appearance of the hills at a distance on its lower Side induced me to call this red Stone river

Confluence of the Powder and Yellowstone rivers

The red rocks in and near this stream induced Clark to call it red Stone river.


red stone

Photo by Clay Schwartz, MBMG

The red-colored stones are chunks of clinker from the Fort Union Formation that washed or fell into the river, which tumbled and smoothed them. Clinker forms when lightning, grass fires, or spontaneous combustion ignites coalbeds, and the adjacent rock— if siltstone or shale — is baked and fused, forming orange, red and yellow “burned rock"


clinker and coal washed


Photos by Ginette Abdo, MBMG

The Powder river at Terry

The next day, July 31, Clark continued downstream about 7 miles past Powder River to present-day Terry, Montana.

Clark July 31, 1806

. . . this high Country is washedinto Curious formed mounds & hills

. . . here the river approaches the high mountainous country on the N W. Side. thosehills appear to be composed of various Coloured earth and Coal without much rock. I observe Several Conical pounds which appear to have been burnt. this high Country is washed into Curious formed mounds & hills and is cut much with reveens

Terry Badlands


The high Country is entirely
bar of timber. great quantities of Coal or carbonated wood is to be seen in every Bluff and in the high hills at a distance on each Side.

Photo courtesy of Wayne Mumford (www.waynemumford.com)


Fort Union Formation in Terry Area








The rugged hills on the N.W. Side of the Yellowstone are the Terry badlands carved out of the Fort Union Formation.The darker, patterned, flat area south of the river is flood plain alluvium, formed by modern erosion and flood deposits.


These mounds are capped with erosion-resistant clinker — shale and siltstone that were baked when the underlying coalbeds burned

Photos by Ginette Abdo, MBMG


The dark layers are coal formed 65-55 million years ago from the decay of vegetation in a swampy area

/. . . I observe Several Conical pounds [mounds] which appear to have been burnt.

. . . great quantities of Coal or carbonated wood is to be seen in every Bluff and in the high hills at a distance on each Side.

Lewis and Clark sometimes called the coal of eastern Montana and western North Dakota “carbonated wood” because it contained the remains of the woody plant material. This low-grade coal is lignite.


Sediment . . . erosion . . .badlands...

The curious landscape Clark described resultedfrom erosion of the Fort Union Formation.

location mapAltitudes near Terry range from 2200 to 2500 feet above sea level; the climate is semi-arid. About 65–55 million years ago, however, vegetation grew abundantly here in a moist, subtropical climate near sea level.

Rivers flowing eastward from the mountains toward the inland sea deposited sand, silt, and mud.


woodlands, grass lands, swamps
Image courtesy of The Field Museum, photographer John Weinstein ©1991


Woodlands, grasslands, and swamps were interspersed in the area. As plants in the swamps died their remains accumulated and slowly turned to peat.

When the rivers meandered or flooded, layers of sand, silt and clay buried the partially decomposed vegetation (peat) in the swamps.

Through geologic time the clay, silt and sand became mudstone, siltstone, and sandstone, respectively. The peat became coal.

Terry Badlands



Sandstone and clinker of the Fort Union Formation tend to resist erosion; they often cap hills and buttes in the area. The finer-grained siltstone and mudstone of this formation erode more easily.

Rivers and seasonal streams cut through the flat-lying rocks, forming the canyons, ravines, gullies, and hoodoos typical of a badland landscape.




Photo by Ginette Abdo, MBMG