Strata of lignite in the Hell Creek beds, east of Fort Peck Dam in northeastern Montana.
After the captains turned west up the Missouri River from Fort Mandan on April 7, 1805, descriptions of coal beds and burnt appearances became common in their journals. On April 9 Lewis noticed "many horizontal statas of carbonated wood, having every appearance of pitcoal at a distance, were seen in the face of these bluffs. These stratas are of unequal thicknesses from 1 to 5 feet, and appear at different elevations above the water, some of them as much as eighty feet." These descriptions reach their fullest in the area of the later fur-trading post of Fort Union, near the confluence of the Missouri and Yellowstone. It is here that the Tongue River Member of the Fort Union Formation "rises" above river level and displays its many beds of coal.
Lewis and Clark sometimes called this coal "carbonated wood" because sometimes they could see the outlines of woody stems and other plant plant remains. Coal geologists call it lignite, but Lewis and Clark were essentially correct in their description.
On April 29, 1805, when the Expedition was passing the heart of the Girard coalfield (in today's Richland County, in northeastern Montana), Lewis gave one of the Expedition's best descriptions of the coal in the formation and its relationship to clinker:
There is more appearance of coal today than we have yet seen. The stratas are 6 feet thick in some instances; the earth has been birnt in many places, and always appears in stratas on the same level with stratas of coal.
A few miles upstream from where Lewis wrote that, the Fort Union Formation lies farther from the river and was not observed by the Expedition. However, on the seventeenth of May, near present-day Seven-Blackfoot Creek, Clark saw some coal from the Fort Union Formation that had been "thrown out by the floods."
Other coal and burnt appearances that the Expedition saw upriver from this point belonged to the Judith River and Eagle formations, which are even older than the Fort Union Formation.
Funded in part by a grant from the Montana Committee for the Humanities