He was correct.
He recorded several observations of the Fort Union Formation among the red hills near Terry, Montana, on July 31. Five and one-half miles below the previous night's camp at the mouth of the Powder River, he "passed under a high Bluff of different coloured earty on the Lard. side." Four miles farther on he observed "high Coal bluffs on the Lard Side." One and one-half miles farther he reached "a red bluff on the Lard. side." Another eleven miles took him past "Steep Coal banks on each side" of "Coal River"--which Sheheke had called Oak Tar pon er, and we now call O'Fallon Creek.1 Eleven miles beyond that he saw "low coal bluffs on the Stard Side." He had navigated through the Tullock, Lebo, Tongue River, and Ludlow members of the Fort Union Formation, where the coal beds have been burned, and have baked the adjacent mudstones into a most striking brick-red hue that slashes boldly across the faces of towering bluffs.2
Lewis, traveling back down the Missouri in late July and early August 1806, was making about 70 river miles per day and did not record much in the way of geology. Nevertheless, his journal makes it clear when he reached the area underlain by the Fort Union Formation.
On 7 August 1806 Lewis wrote that near the mouth of Martha's River--today, Big Muddy Creek, which Clark named in honor of the "Selebrated" but anonymous "Miss M F"--"we meet with the first appearance of the coal, burnt hills and pumice stone. These appearances seem to be coextensive." This area of lignite actually extends for about ten miles upriver from Big Muddy Creek, but the bedrock is deeply covered with glacial deposits, so little of it is exposed.After leaving Fort Mandan en route home in the late summer of 1806, the Expedition was in too much of a hurry to note geographic and geologic features already described on the outbound trip. The Expedition exited the area underlain by the Fort Union Formation late in the day on 19 August 1806.
1. The stream labeled Gibsons River on Atlas maps 121 and 123, Gibson River on Atlas map 125 and Gibsons Creek on Atlas map 126 actually is present-day Cabin Creek. "Coal River" in Clark's journal narrative refers to present-day Cabin Creek, which joins the Yellowstone about ten miles downstream from Fallon Creek. See Gary E. Moulton, ed., Atlas of the Lewis & Clark Expedition (Lincoln: University of Nebraska Press, 1983).
2. Of the captains' many references to the sightings of coal along their route, this is the only reference that Nicholas Biddle included in his edition of the journals (1814).
Funded in part by a grant from the Montana Committee for the Humanities.