Lewis and Clark in Montana — a geologic perspective

"The object of your mission is to explore the Missouri river, & such principal stream of it, as, by it's course & communication with the water of the Pacific ocean may offer the most direct & practicable water communication across this continent, for the purposes of commerce.

... Other objects worthy of notice will be; the soil & face of the country ... the animals...the remains or accounts of any which may be deemed . . . extinct; the mineral productions of every kind; but more particularly metals, limestone, pit coal, & saltpetre, salines & mineral waters, noting the temperature of the last, & such circumstances"

Excerpt from instructions written by President Thomas Jefferson to Meriwether Lewis, June 20, 1803

The science of Geology was in its infancy at the time of the Lewis and Clark Expedition. Neither Lewis nor Clark had any training in geology, mining, or mineralogy. The geologic terms they used, however, reveal much about their understanding (and misunderstanding) of geology. Their journal entries name several rock types: chalk, flint, flintstone, freestone, granite, lava, limestone, marble, marl, pumicestone, sandstone, sandrock, slate, slate stone, and slate rock. Generally, the captains identified these rock types correctly – but not always. The captains were acquainted with names of many minerals, nevertheless most of the minerals and salts that they described were identified incorrectly.

Lewis and Clark sometimes ascribed colors to rocks that probably resulted from conditions of sunlight, shadow or vegetation rather than to actual color of the rocks themselves. Both captains had a good command of geographic terms and used most of them in their current meaning. Both also seemed to have had some concept of geologic time because they occasionally used the phrase, “in some former age” when referring to river changes and certain geographic features.

Both Lewis and Clark had more of an understanding of stream erosion and deposition than did most people of the time, and the geologic observations they made during the expedition were among the finest of the day in America.

Little information has reached the public detailing the geography, geology, minerals or fossils that Lewis and Clark described in their journals. These pages depict and explain some of the more important geological observations and navigational aspects that the expedition recorded while within the present state of Montana.

A PDF file for the Lewis and Clark section only is available to download (2.7 Mb). You can also purchase these pages as posters (MC21A-I) from our online catalog.

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Text by Bob Bergantino and Ginette Abdo; images as cited; graphics by Susan Smith; web adaptation by Nancy Favero.