Geology of the Breaks

In Thomas Jefferson‘s instructions to Meriwether Lewis—what Paul Russell Cutright has called his “blueprint for discovery”—the only geological subjects considered worthy of note were paleontology and mineralogy.^[2]Paul Russell Cutright, Lewis and Clark: Pioneering Naturalists (Urbana: University of Illinois Press, 1969), 1. Jefferson told Lewis to be on the lookout for “the remains or accounts of any [animals] which may be deemed rare or extinct; the mineral productions of every kind; but more particularly metals, limestone, pit coal, & saltpetre; salines & mineral waters . . . [and] volcanic appearances.”^[3]Donald Jackson, ed., Letters of the Lewis and Clark Expedition, With Related Documents, 1783-1854, 2 vols. (Urbana: University of Illinois Press, 1978), 1:63. See also Donald Jackson, Thomas … Continue reading

Given Jefferson’s instructions, it’s not surprising that the journal entries relating to earth science have been judged by some scholars, including Donald Jackson, as “routine and unexciting.”^[4]Donald Jackson, Among the Sleeping Giants (Urbana: University of Illinois Press, 1987), 15.

Jackson’s take on the matter aside, a careful reading of the captains’ geological entries yields much detailed and generally accurate information. Assume for a moment that you are a mid-19th-century geologist planning to survey north central Montana between the Missouri Breaks and the White Cliffs. The Corps of Discovery made its way up this stretch of the Missouri River during the last week of May 1805, and you would find the journals a good start for understanding its geology. As a professional geologist and a Lewis and Clark enthusiast I have come to know this country well and am impressed by what the captains had to say about it, as suggested by the following journal excerpts and commentary.

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, 25 May 1805^[5]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 … Continue reading

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.

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, 26 May 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.

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, 26 May 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.

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, 27 May 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.^[6]Geologists use the term “formation” to describe a lithologically distinct body of rock. 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.

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, 28 May 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, 29 May 1805

On 28 May, 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.^[7]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 … Continue reading

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, 30 May 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.^[8]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. … Continue reading

The bluffs of the river rise to the hight of from 2 to 300 feet and in most places nearly perpendicular; they are formed of remarkable white sandstone which is sufficiently soft to give way readily to the impression of water; two or thre thin horizontal stratas of white free-stone, on which the rains or water make no impression, lie imbeded in these clifts of soft stone near the upper part of them.

—Lewis, 31 May 1805

The Corps of Discovery had now entered the White Cliffs area. Lewis is describing the Virgelle Sandstone, a “member” or subunit of the Eagle Sandstone Formation.^[9]The Eagle Formation was named by Walter Harvey Weed in 1899 for Eagle Creek (U.S. Geological Survey Geological Atlas, Fort Benton folio, No. 55). Eagle Creek is a tributary of the Missouri entering … Continue reading As Lewis points out, the layers of this sandstone vary in hardness and therefore erode at different rates, creating the columns, parapets, and pyramids that inspired his famous description (quoted below) of “seens of visionary inchantment.” The Virgelle’s distinctive creamy whiteness comes from volcanic ash that blanketed ancient barrier islands and off-shore sandbars when these rocks were forming during the Cretaceous.^[10]The Virgelle Member is named for a nearby river town. Another soft, Cretaceous sandstone would provide Clark, on his return down the Yellowstone River, with a rough-hewn canvas to carve his … Continue reading

Lewis continues:

the earth on top of these Clifts is a dark rich loam, which forming a graduly ascending plain extends back from 1/2 a mile to a mile where the hills commence and rise abruptly to a hight of 300 feet more.

From this description a geologist would recognize the obvious change from sandstone (which forms steep cliffs) to shale (which weathers to form long slopes) in the White Cliffs area.

A little further, Lewis writes,

The water in the course of time in decending from those hills and plains on either side of the river has trickled down the soft sand clifts and woarn it into a thousand grotesque figures, which with the help of a little immagination and an oblique view at a distance, are made to represent eligant ranges of lofty freestone buildings, having their parapets well stocked with statuary; collumns of various sculpture both grooved and plain, are also seen supporting long galleries in front of those buildings; . . . some collumns standing and almost entire with their pedestals and capitals; other retaining their pedestals but deprived by time or accident of their capitals, some lying prostrate an broken othes in the form of vast pyramids of connic structure bearing a sereis of other pyramids . . . . nitches and alcoves of various forms and sizes are seen at different hights as we pass . . . the thin stratas of hard freestone intermixed with the soft sandstone seems to have aided the water in forming this curious scenery. As we passed on it seemed as if those seens of visionary inchantment would never have [an] end.

This passage is justly celebrated for its flights of romantic fantasy, but the line about ‘thin stratas of hard freestone” shows how well Lewis understood how erosion works. From this passage a geologist would recognize that the sandstone is weakly cemented and therefore vulnerable to the erosive effect of water percolating through it. As Lewis suggests, intermingled layers of harder “freestone” rocks (also composed of sandstone, but with a slightly different composition) erode less readily. By “freestone” Lewis may also have been referring to certain circular, iron-rich concretions for which the Virgelle Member is noted. These concretions act as protective capstones and are chiefly responsible for the pulpits, toadstools, and other unusual features.

Lewis goes on:

for here it is too that nature presents to the view of the traveler vast ranges of walls of tolerable workmanship, so perfect indeed are those walls that I should have thought that nature had attempted here to rival the human art of masonry . . . . These walls rise to the hight in many places of 100 feet, are perpendicular, with two regular faces and are from one to 12 feet thick, each wall retains the same thickness at top which it possesses at bottom. The stone of which these walls are formed is black, dence and dureable . . . . these are laid regularly in ranges on each other like bricks, each breaking or covering the interstice of the two on which it rests . . . . These walls pass the river in several places, rising from the water’s edge much above the sandstone bluffs, which they seem to penetrate; thence continuing their course on a streight line on either side of the river through the gradually ascending plains, over which they tower to the hight of from ten to seventy feet untill they reach the hills, which they finally enter and conceal themselves. these walls sometimes run parallel to each other, with several ranges near each other, and at other times interscecting each other at right angles, having the appearance of the walls of ancient houses or gardens.

These walls, technically termed dikes, are composed of an igneous rock known as shonkinite.^[11]Named after the town of Shonkin, some 30 miles southwest of the White Cliffs. According to the laws of stratigraphic nomenclature, distinct rock formations are named for the nearest prominent … Continue reading They were formed some 50 million years ago, when molten rock deep within the earth rose to fill vertical fractures within the Eagle Formation.^[12]The number of “walls” depicted on the expedition-related maps varies. Three eastern walls and one western “rock” are shown on the Lewis and Clark rough draft (Moulton, Atlas, … Continue reading Later, water carved away the softer surrounding rock, leaving these freestanding structures. Lewis’s deliberate narrative accurately conveys their density and salient nature, and his description of their bricklike jointing pattern is right on the mark.^[13]In the following sentence Lewis notes that he “walked on shore this evening and examined these walls minutely and preserved a specimine of the stone.” We don’t know if the specimen … Continue reading

The detailed geological and physical descriptions of the Missouri River Breaks and White Cliffs, paraphrased and better punctuated, survived nearly intact in the Biddle version of the journals (published in 1814) and thus were available to any early geologist fortunate enough to find or borrow a copy. It would be a half century before geological exploration of the Missouri River Breaks and White Cliffs area would begin in earnest. The pioneering geologist of the American West, Ferdinand Vandiveer Hayden, explored the Missouri as far up as Fort Benton in 1854–55, and beginning in 1856 he published in the Proceedings of the Philadelphia Academy of Science a series of papers describing its geology.^[14]Ferdinand Vandiveer Hayden (1829-1887) was a trailblazing geologist and explorer of the West. He directed the U.S. Geological Survey of the Territories, which focused on Nebraska, Wyoming (including … Continue reading We know that Hayden was familiar with the captains’ journal observations, for he refers to their description of the region (almost certainly derived from Biddle) in his 1860 paper “A Geological Sketch of the Estuary and Fresh-water Deposits of the Bad Lands of the Judith,” published in Transactions of the American Philosophical Society—a most appropriate venue considering that Jefferson and Lewis were members of that distinguished society. Hayden remarks that the captains gave an “accurate description of the physical features of this remarkable region,” particularly in the section they named the “Stone Walls.”^[15]Ferdinand V. Hayden, Transactions of the American Philosophical Society, 1860, 123-138. By naming the Judith River Formation in 1871 after Clark’s Judith River, Hayden also helped ensure that the captains’ legacy would live on in geological literature.

It may never be known how much preliminary knowledge Hayden or his colleagues in the U.S. Geological Survey of the Territories gained from the journals of Lewis and Clark before their forays into Montana. And it must be recognized that the captains were not geologists, were believed to have carried only one book that had anything to say about geology, entertained no speculations on the age of the rocks, and according to both Cutright and Jackson recorded less about geology as the expedition proceeded westward.^[16]Cutright, Lewis and Clark, 57, and Jackson, Jefferson and the Stony Mountains, 197. Yet the observations they did make remain revealing and useful. Speaking from experience, I know that anyone preparing for a pioneering survey and mapping exercise like Hayden’s would have been grateful for this advance knowledge about a region of the West that was terra incognita for geologists.

The White Cliffs is a High Potential Historic Site along the Lewis and Clark National Historic Trail managed by the U.S. National Park Service. It is part of the Upper Missouri River Breaks National Monument managed by the Bureau of Land Management.—ed.