Missouri River Breaks: "high broken and rockey"

masthead saying 'We Proceeded On'
Reprinted from We Proceeded On1

Missouri River Breaks

Bare and steep cliffs with rocks tumbling into the Missouri River

Photo by John W. Jengo

This rugged topography, located downstream of the Missouri River confluence with the Judith River, is typical of the Missouri Breaks.

Layered bank near Kipp Recreation Area

Layered bank above the Missouri River

Photo © July 25, 2013 by Kris Townsend

Dog Creek and Missouri River confluence

Large valley with barren hills

Photo © July 24, 2013 by Kris Townsend

This view is northeast from a hill above the Judith River valley.

—editor, KT

Judith River Valley

Wide, green valley surrounded by barren hills

Photo © May 24, 2016 by Kris Townsend

The Judith River Basin provided a respite from the stark topography of the Missouri River Breaks.

—editor, KT

Muddy, steep bank

Steep, brown bank with loose rocks

Photo © May 22, 2016 by Kris Townsend

When it rains in the Missouri Breaks, the banks become extremely slick. Towing the heavy dugouts, the Expedition passed several muddy banks like this one on May 30, 1805.

—editor, KT

May 25, 1805: "high broken and rockey"

The Country on either hand is high broken and rockey; the rock is either soft brown sand stone covered with a thin strata of limestone, or a hard black rugged grannite, both usually in horizontal stratas and the Sandy rock overlaying the other.—Salts and quarts still appear, some coal and pumice stone also appear.

—Lewis, May 25, 18052

Lewis was wrong in stating that the formation includes granite, but he was correct about the sandstone, limestone, and coal. Sandstone and limestone are sedimentary rocks formed, respectively, by deposits of sand and calcite laid down over time. This can occur in either a continental or marine environment. The presence of coal—formed on land from ancient vegetation—indicates that the sedimentary rocks along this part of the Missouri are continental in origin. Long after Lewis and Clark passed this way, geologists determined that these rocks were laid down primarily during the Cretaceous Period, 135 million to 65 million years ago, when dinosaurs roamed Montana.

May 26, 1805: Concreted Shells

the black rock has given place to a very soft sandstone which appears to be washed away fast by the river, above this and towards the summits of the hills a hard freestone of a brownish yellow colour shews itself in several stratas of unequal thicknesses frequently overlain or incrusted by a very thin strata of limestone which appears to be formed of concreted shells.

—Lewis, May 26, 1805

Here Lewis describes the variability of different rock types along this part of the Missouri. In the phrase "concreted shells" we find one of the captains' few references to fossils. From Lewis's description, a geologist can easily visualize the impossibly dense, tightly packed, cemented mass of fossil oyster shells that make up these so-called "shell hash" deposits.

May 26, 1805: Troublesome Hills

The country which borders the river is high broken and rocky, generally imbeded with a Soft Sand Stone higher up the hill the Stone is of a brownish yellow hard and gritty those Stones wash down from the hills into the river and cause the Shore to be rocky &c. which we find troublesom to assend.

—Clark, May 26, 1805

The first part of Clark's entry echoes Lewis's entry of the day before. His description of stones washing into the river indicates the active erosion one finds along this part of the Missouri. A geologist would correctly infer from the passage that this stretch of river is in the early stages of canyon formation.

May 27, 1805: Judith River Formation

the bluffs are very high steep rugged, containing considerable quantities of stone and border the river closely on both sides; . . . great quantities of stone also lye in the river and garnish it's borders, which appears to have tumbled from the bluffs where the rains had washed away the sand and clay in which they were imbeded. the bluffs are composed of irregular tho' horizontal stratas of yellow and brown or black clay, brown and yellowish white sand, of soft yellowish white sand stone and a hard dark brown free stone, also of large round kidneyformed and irregular seperate masses of a hard black Iron stone, which is imbeded in the Clay and sand . . . some coal or carbonated wood still makes it's appearance in these bluffs, pumicestone and birnt hills it's concommutants also are seen.

—Lewis, May 27, 1805

Lewis provides a lot of perceptive detail here, particularly about the thickness and varied coloration of the rocks now defined as the Judith River Formation.3 It is easy to visualize, from his description, the sandstone layers alternating with layers of siltstone and shale. The layers' varying rates of erosion produce the stark, badland topography of the Missouri Breaks. Lewis notes the continuing presence of coal seams but makes clear that these beds are less prominent than the sandstone and shale. He also astutely observes that the "kidneyformed and irregular" ironstone (a term still used to describe a very hard, iron-rich sedimentary rock) are imbedded within the formation.

May 27 and 28, 1805: Judith River Basin

here the hills recede from the river on both sides, the bottoms extensive particularly on the Stard. Side where the hills are comparitively low and open into three large vallies which extend for a considerable distance in a Northwardly direction.

—Lewis, May 28, 1805

at this place the hills again approach the river closely on both sides, and the same seen which we had on the 27th and 28th in the morning again presents itself.

—Lewis, May 29, 1805

On May 28 the explorers entered a relatively open part of the river, but the next day the hills again closed in. The identification of this break in the cliff-dominated Missouri would alert a geologist to faults or other structural features that are natural channels favoring the formation of river beds. In fact, this structurally complex region is pervaded by faults, and the Judith River, a major tributary of the Missouri, enters the river on the south side.4

May 30, 1805: Tilted Formations

the banks and sides of the bluff were more steep than usual and were now rendered so slippery by the late rain that the men could scarcely walk . . . . the earth and stone also falling from these immence high bluffs render it dangerous to pass under them.

—Lewis, May 30, 1805

This passage makes it easy to appreciate the proximity of the sheer rock cliffs. The description of these steep, confining cliffs is in marked contrast to the entry of just two days before about receding hills and open bottomlands. The changing topography results from the different rocks exposed at the waterline. This would suggest two possible scenarios. One is that a steep river gradient is cutting through different rock formations. The other is that geological forces have tilted the formations, which are normally horizontal, toward the river's flow, thereby increasing its cutting angle. Mapping by geologists shows, in fact, that these formations are tilted.5

 
  • 1. John W. Jengo, "high broken and rocky:" Lewis and Clark as geological observers, We Proceeded On, Volume 28, No. 2 (May 2002), the quarterly journal of the Lewis and Clark Trail Heritage Foundation. Page titles, subheadings, and graphics have been added. The original printed format is provided at http://lewisandclark.org/wpo/pdf/vol28no2.pdf#page=24.
  • 2. Gary E. Moulton, ed., The Journals of the Lewis & Clark Expedition (Lincoln: University of Nebraska Press, 1991), 4:195. All quotations or references to journal entries in the ensuing text are from Moulton, Vol. 4, by date, unless otherwise indicated. Here and elsewhere in quoted journal sections referring to sedimentary bedding, Lewis uses "strata'" as the singular and "stratas" as the plural. In the explanatory text the author uses "strata" (correctly) as the plural and "layer" as the singular (avoiding the grammatically correct but relatively unfamiliar term "stratum").
  • 3. Geologists use the term "formation" to describe a lithologically distinct body of rock.
  • 4. Clark named the river for his cousin and future wife, Judith (Julia) Hancock. Lewis named it the Big Horn, but Judith prevailed. Today's Big Horn River is a tributary of the Yellowstone. See Moulton, Journals, 220n, and Atlas, map 52.
  • 5. In sedimentary formations, the age of the rock layers, or strata, varies with depth—the older strata, which were laid down first, are toward the bottom and the younger strata are toward the top. Travelers going up a river that cuts through a horizontal sedimentary formation, therefore, are exposed to increasingly younger rocks. The formations along this part of the Missouri are not horizontal but tilt toward the east, in the same direction as the river's flow but at an angle significantly steeper than the river gradient. Therefore (and somewhat counterintuitively), travelers are exposed to older rocks as they ascend the river between the Missouri Breaks and the White Cliffs.