Photo by the author
The captains correctly identified extensive exposures of the Niobrara Formation, a relatively soft and pure form of limestone, in their numerous descriptions of "white earth," "white hard clay," "white clay marl," and "chalk" between August 26 and September 1, 1804.
White Clay 'Marl'
One of the most distinct changes in the geological terrain along the expedition route was immediately recognized by William Clark. He was accurate in describing the Cretaceous age Niobrara Formation as a chalk deposit:2
[August 26, 1804:] a Clift of White earth on the L. S of 2 ms. in length.
[August 27, 1804:] passed a white Clay marl or Chalk Bluff under this Bluff is extensive
I discovered large Stone much like lime incrusted with a Clear Substance which I believe to be Cabalt, also ore is imbeded in the Dark earth, resembling Slate much Softer.
[August 28, 1804:] passed a Short White Bluff of about 70 or 80 feet high.
Understanding Clark's Descriptions
Chalk is a relatively soft and pure form of limestone and is composed primarily of calcium carbonate (CaCO3), in the form of spherical skeletons called cocospheres from a group of chalk forming plankton (single-celled algae).3
Certain strata of chalk are white for a very different reason than the bleaching of the sandstones the captains encountered at the White Cliffs of the Missouri.4 White chalk is essentially free from other sediments or minerals that would discolor it. It is thought that for this to occur, the sedimentary paleo-environment of chalk deposits had to be in clear marine seas far from adjacent landmasses, keeping them isolated from eroded sediments that would affect their composition and color.
Because the Niobrara Formation is a bluff-forming horizon, its most accessible modern exposures are along road cuts that descend onto the flood plain of the Missouri, such as 563 Avenue northwest of St. Helena, Nebraska. Here, the formation is comprised of both massive exposures of chalk and intervals of chalky shale that appeared to have been "chinked" (like old-fashioned mortar) with fragments of medium gray to white chalk. These outcrops remind the viewer of Clark's appropriate use of the word "marl," defined as a loose, well-blended combination of clay and calcium carbonate.5
As far as Clark's "large stone" is concerned, flint nodules can easily be found in the Niobrara Formation; it has been surmised that these nodules developed, probably after multiple rounds of dissolution and re-crystallization, from the siliceous skeletons of organisms such as sponges.
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, "'Blue Earth,' 'Clift of White' and 'Burning Bluffs'," We Proceeded On, Volume 37, No. 1 (February 2011), the quarterly journal of the Lewis and Clark Trail Heritage Foundation. Editorial additions include page titles, side headings, and graphics to assist the web-based reader. The original printed format is provided at http://lewisandclark.org/wpo/pdf/vol37no1.pdf#page=8.
- 2. Moulton, Journals, 3:18, n2.
- 3. Robert L. Bates and Julia A. Jackson, eds., Glossary of Geology, (Falls Church, Virginia: American Geological Institute, 1980), 104.
- 4. Jengo, "Mineral Productions of Every Kind," 207, n50.
- 5. Containing 35-65% clay and 65-35% carbonate according to Francis J. Pettijohn, Sedimentary Rocks, (New York: Harper & Brothers, 1957), 410.