'Blue Clay' and Bluffs 'on Fire'

masthead saying 'We Proceeded On'

Reprinted from We Proceeded On1

August 22–24, 1804 Route Map (Detail)

Map showing the Missouri River channel has moved since 1804

Base Map Data © 2009 Google;

Detailed route map of the August 22–24, 1804 expedition traverse up the Missouri River in northeastern Nebraska. By mapping the 1804 course of the river, the present-day location of the "Ionia Volcano," and the position of the "Rugged Burning Bluffs" observation made by William Clark. west-northwest of present-day Lime Creek, it is evident that Ionia Volcano locale has been incorrectly linked with the essential expedition observation that "Those Bluffs appear to have been laterly on fire, and at this time is too hot for a man to bear his hand in the earth."

Too Hot to Touch

Among the most significant scientific remarks to be recorded on the entire expedition, and one that would generate great interest from the scientific community back in Washington upon the expedition's return, was Clark's observation from August 24th:

Commencement of a blue Clay Bluff of 180 or 190 feet high on the L.S. Those Bluffs appear to have been laterly on fire, and at this time is too hot for a man to bear his hand in the earth at any debth, gret appearance of Coal. An emence quantity of Cabalt or a Cristolised Substance . . . on the face of the Bluff.

Carlile Shale

Clark's "Blue or Dark earth"2 or "blue Clay Bluff' notations typically refer to the Cretaceous age Carlile Shale.3 This observation is supported by Ordway's journal, which says that this deposit had great quantities of "mineral Substance," and would undoubtedly have included selenite.4

On-site examiners should note that it is difficult to closely examine the "landlocked" bluffs of the Carlile Shale today. Only a few feet remain visible of the "blue clay" facies of the Carlile Shale at the Ionia Volcano location; colluvium has buried nearly 70% of the outcrop. By contrast, at the time the captains encountered the Carlile Shale along the river, any material that slumped from higher elevations was continually being swept away by the Missouri River. This allowed much fuller exposures of the Carlile Shale to be observed.

Marcasite + Oxygen + Water = Heat

The fascinating chemical reaction Lewis and Clark saw in 1804 begins with iron sulfide minerals present in the Carlile Shale.5 Iron sulfide (FeS2), particularly in the form of the mineral marcasite,6 reacts readily with oxygen (O2) and water (H2O) to produce ferrous iron (Fe+2), and through a follow-up reaction with oxygen and hydrogen, ferric iron (Fe+3) is produced. Not only is this reaction "exothermic" (releasing energy in the form of heat), it also has a positive feedback loop component; when ferric iron is produced, it can also oxidize iron sulfide minerals. This process has a chain reaction component; the hydrogen (H+) that is thrown off also lowers the pH, which creates an environment for acid-loving microorganisms (or acidophiles) to participate in and accelerate the reaction (by "up to few orders of magnitude" under certain conditions).7

Interestingly, museums constantly wrestle with the degradation of iron sulfide minerals in their collections because of this same phenomenon. It is likely the reason why mineralogist Adam Seybert was already noting decomposition of some of Lewis's pyrite/marcasite specimens after their arrival in Philadelphia. For example, Seybert documented that the Fort Mandan mineralogical specimen No. 41, collected on August 23, 1804, had "Pyrites decomposed" and other iron sulfide samples in the collection were "in a state of efflorescence."8

The unique feature here is the interaction between the iron sulfide and clay minerals that comprise the Carlile Shale. The clay minerals absorb and retain water, critically driving the reaction.9 If lignite (the lowest grade of coal) is present, it will ignite and generate its own heat as well.10 These reactions are the source of the "burning bluffs" so famously described by Clark, and confirmed by the ever-diligent Sgt. Ordway, who remarked that the "burning bank or Bluff" had a "sulpheras Smell."11

Was it the Ionia Volcano?

Mulberry Bend State Wildlife Management Area

Clark's bluffs 'on fire'

Forested hills along the Missouri River

Photo by the author

Clark's bluffs "on fire" observation on August 24, 1804 should actually be applied to the rock exposures west-northwest of the Route 15 bridge at Mulberry Bend SWMA, upriver of Missouri River's former confluence with Lime Creek, which corresponds to an area where the Carlile Shale was broadly exposed to, and literally on the edge of, the 1804 position of the river.

The essential part of Clark's journal entry, particularly variations on the phrase "Those Bluffs appear to have been laterly on fire, and at this time is too hot for a man to bear his hand in the earth," has come to be extensively used in historical signage,12 various travel and geological guidebooks,13 and merits important footnotes in the definitive editions of the journals.14 Yet unfailingly, the description is ascribed to the so-called Ionia Volcano, a narrow exposed bluff located approximately 3.5 miles northeast of Newcastle, Nebraska. The Ionia Volcano, though, is not the only Carlile Shale bluff in this locale so it was worth questioning how this particular bluff came to be identified as "the" pseudo-volcano that Clark was describing. That this general area was the site of pseudo-volcanic activity seems indicated by its name, "Volcano Hill". But in 1804, the river was at least one mile away according to Martin Plamondon's cartographic reconstruction; thus, it can be confidently concluded that the Missouri River was not actively undercutting this bluff when the expedition passed by on August 22, 1804.15

Expedition maps of the painstaking haul up the Missouri River during this time period clearly reveal that they passed the Ionia Volcano locale on August 22nd, and camped on the opposite side of the river.16 Neither the captains nor the other extant journal keepers (Ordway, Whitehouse, and Gass) noted any volcanic occurrences, suggesting that the Ionia Volcano was not the subject of Clark's subsequent observations.17

If Clark was recording his August 24, 1804 Field Notes observations in the present tense (and the geological details on expedition maps18 is accurate, Clark's bluffs "on fire" observation on August 24, 1804 should actually be applied to the rock exposures west-northwest of the Route 15 bridge at Mulberry Bend SWMA, upstream from the river's former confluence with Lime Creek (see map).

Clark was quite specific that the "Blue Clay Bluffs" that "have been latterly on fire" commenced 2.5 miles upriver from the expedition's August 23rd encampment and that they traveled along these "verry hot" bluffs for 1.75 miles.19 According to geological maps,20 the burning bluffs location correlates precisely with an area where the Carlile Shale was broadly exposed to, and literally on the edge of, the 1804 position of the river, thus providing William Clark with an opportunity to examine firsthand that these bluffs were "too hot for a man to bear his hand in the earth."21

  • 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, 3:14.
  • 3. Moulton, Journals, 3:15, n6. The Carlile Shale "blue earth" facies can be technically described as a "very dark bluish gray."  It does appear distinctly bluish in outcrop.
  • 4. Moulton, Journals, 9:43.
  • 5. Joseph Nicolas Nicollet, a French scientist, mathematician, and topographer, deduced this relationship following his April 4-June 12, 1839 retracing of Lewis and Clark's route up the Missouri River. Nicollet stated that "many have erroneously supposed that volcanoes existed on the Upper Missouri. This, however, is a mistake. The smoke and pseudo-pumice . . . proceed from the same source, the ignition of the iron pyrites and lignite, which are found in great abundance in the plastic clay". The American Journal of Science and Arts (October 1843), 45:154-155.
  • 6. Although chemically identical to pyrite, marcasite has been found to oxidize more readily than pyrite due to its crystal structure (orthorhombic rather than cubic or isometric).
  • 7. B.E. Taylor and M.C. Wheeler, "Sulfur- and Oxygen-Isotope Geochemistry of Acid Mine Drainage in the Western United States: Field and Experimental Studies Revisited," in Charles N. Alpers and David W. Blowes, eds., Environmental Geochemistry of Sulfide Oxidation: Developed from a Symposium sponsored by the Division of Geochemistry, Inc., at the 204th National Meeting of the American Chemical Society, Washington, D. C., August 23-28, 1992, ACS Symposium Series 550, (Washington, DC: American Chemical Society, 1994), 481-514.
  • 8. Moulton,]ournals, 3:476-477. Seybert's observation of efflorescence suggests he was seeing some kind of incrustation or powdery deposit on the surface of the specimens, which normally is comprised of an aggregate of halotrichite and melanterite; see Charles B. Sclar, "Decomposition of Pyritized Carbonaceous Shale to Halotrichite and Melanterite," The American Mineralogist, Vol. 46, May-June 1961, 754-756.
  • 9. S.L. Borek, "Effect of Humidity on Pyrite Oxidation," in Alpers and Blowes, eds., Environmental Geochemistry of Sulfide Oxidation, 31-44.
  • 10. Diffendal, Geology of the Great Plains, 30.
  • 11. Moulton, Journals, 9:43.
  • 12. Such as the Nebraska Historical Marker "The Ionia Volcano" located on Route 12 in Pfister Park on the eastern approach to Newcastle at the intersection of Rt. 12 (Annie Street) and Martha Street.
  • 13. Such as Thomas Schmidt, National Geographic's Guide to The Lewis and Clark Trail, (Washington, D.C.: The National Geographic Society, 1998), 36-37; and Diffendal, Geology of the Great Plains, 30.
  • 14. Moulton, Journals, 2:506, n1.
  • 15. Plamondon, Lewis and Clark Trail Maps, 1:106. Close proximity to the river would trigger/facilitate the reaction and enable the captains to witness the phenomenon up close. These conditions do not appear to have occurred on August 22, 1804.
  • 16. Gary E. Moulton, ed., Atlas of the Lewis and Clark Expedition, (Lincoln: University of Nebraska Press, 1983), Map 17.
  • 17. Although not recorded in the field notes or notebook journals, the captain did collect a specimen of pyrite on the following day (August 23, 1804) that Lewis stated was "found at the Burning Cliffs," but even here, it's likely that this specimen was collected near present-day Turkey Creek at the closest approach of 1804 Missouri River to the Carlile Shale on the August 23rd traverse, and some four miles west-northwest from the Ionia Volcano; see Fort Mandan mineralogical specimen No. 46 in Moulton, Journals, 3:476.
  • 18. Moulton, Atlas, Map 17. verify that he was), then the Ionia Volcano locale has been incorrectly linked with this essential expedition observation. If the expedition route mapped by Martin PlamondonPlamondon, Lewis and Clark Trail Maps, 1:107-108.
  • 19. Moulton, Journals, 2:506.
  • 20. Burchett, et al., Bedrock Geologic Map, USGS Map 1-1879.
  • 21. Candidates for the true "burning bluffs" include the exposures that can be readily seen along 891 Road west-northwest of the Missouri River's former confluence with Lime Creek and approximately 8,500-11,150 feet east-southeast of where Ames Creek emerges from the uplands.