Geological Observations and Commentary

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

Map showing the white cliffs and breaks areas

The Corps of Discovery passed through the Missouri Breaks-White Cliffs area in May 1805 on its outward-bound journey. This part of the Missouri River remains one of the most remote sections of the L&C Trail.

Clark's Map of the Missouri Breaks (Detail)

Hand-drawn map of the Missouri River Breaks

Beinecke Rare Book and Manuscript Library, Yale University

Clark's "Big Horn River" is the Judith and "Slaughter River" is now Arrow Creek.

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 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

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

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.

  • 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
  • 2. Paul Russell Cutright, Lewis and Clark: Pioneering Naturalists (Urbana: University of Illinois Press, 1969), 1.
  • 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 Jefferson and the Stony Mountains (Urbana: University of Illinois Press, 1981), 32.
  • 4. Donald Jackson, Among the Sleeping Giants (Urbana: University of Illinois Press, 1987), 15.