Hills rising behind white cliffs
Photo © May 22, 2016 by Kris Townsend
Lewis may have had this view of the White Cliffs in mind when he described them as overlain by "a dark rich loam" extending back to where "the hills . . . rise abruptly." The creamy white sandstone is in marked contrast to the dark, overlying rocks and shonkinite dike walls.
Photo © May 22, 2016 by Kris Townsend
Photo © May 21, 2016 by Kris Townsend
The striking columns, pulpits, minarets, pedestals, toadstools, and other unique features of the White Cliffs, immortalized in Lewis's journal entry of May 31, 1805, owe their creation to caps of iron-rich concretions that partially protect the underlying sandstone from erosion.
Photo © May 22, 2016 by Kris Townsend
May 31, 1805: The White Cliffs
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, May 31, 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.2 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.3
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.
May 31, 1805: "seens of visionary inchantment"
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.
May 31, 1805: "walls of tolerable workmanship"
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.4 They were formed some 50 million years ago, when molten rock deep within the earth rose to fill vertical fractures within the Eagle Formation.5 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.6
John W. Jengo
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.
Articles on this site by John W. Jengo:
- 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. 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 on the north bank near the explorers' campsite of May 31, 1805. Lewis and Clark called it Stonewall Creek, but the name did not prevail. If it had, the rocks forming the White Cliffs now might be known as the Stonewall Formation. This would have caused confusion—at least among geologists—since Lewis and Clark also described "Stone Walls" a little farther upriver that are geologically distinct from the White Cliffs. These are dike features composed of shonkinite, a rare mafic silicate with a unique mineralogical composition, principally blocky crystals of glossy black augite and orthoclase (potassium) feldspar, along with other minerals such as biotite, microcline, olivine, and nepheline.
- 3. 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 signature. Thus, the sandstones of Montana have furnished us with two priceless treasures—Lewis's sublime White Cliffs prose and Clark's bold inscription on Pompeys Pillar, carved July 25, 1806.
- 4. 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 geographical feature at the type locality. Photographs of shonkinite dikes from this region of Montana were reproduced in several early geology textbooks, including Branson and Tarr, Introduction to Geology, 1935. For more on shonkinite, see note 8.
- 5. 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, map 41), five eastern walls and one western "rock" are shown on the finished map copy (Moulton, Atlas, map 53), and four eastern walls and one western wall appear on the Clark-Maximilian Sheet 24 (Moulton, Atlas, map 60). This by no means represents the true number of dikes (they actually number in the hundreds), although the variable orientation of the depicted "walls" on these maps does hint at their spatial complexity. The most consistent feature is the "rock" depicted along the western shoreline of the Missouri, which may correspond to today's Citadel Rock.
- 6. 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 made it back to St. Louis because it is not specifically mentioned in the list of items to be shipped to Washington after the expedition's return. Charles Willson Peale mentions that "some minerals &c." were part of a shipment of effects Lewis sent via New Orleans in 1809, but this shipment should not be confused with the specimens shipped back from Fort Mandan in the spring of 1805 because we know the Fort Mandan specimens were received by Thomas Jefferson in October 1805 and were forwarded to Peale later that month. Some of these specimens were lost, and those that survived were subsequently integrated into the collection of the Academy of Natural Sciences, in Philadelphia. Without their original tags it is impossible to identify them with certainty, although the shonkinite specimen, because of its unique mineralogy, may yet be identified in the academy's collections. See Jackson, Letters, 2:469-470 and 476-478, and 1:260-270; also Moulton, Journals, 3:472-478.