Geology Lesson

Cliff Near the Mouth of Tower Creek

"This Clift is of a redish brown Colour"

Tall, tan and red rock jutting up from a sagebrush covered hill

© James Fazio

Upon reaching Tower Creek in the evening of August 21, 1805, Clark described the cliff near which he camped, relying as usual on his limited experience and his unassisted senses to classify the rocks by color and texture. "This Clift is of a redish brown Colour the rocks which fall from it is a dark brown flint tinged with that Colour. Some Gullies of white Sand Stone and Sand fine & a[s] white as Snow." In fact, no flint exists in or below that cliff. It is an outcrop of argillite, which is extremely hard mudstone. This argillite is probably about one billion years old, give or take a hundred million years or so. It belongs to one of the several Belt formations, which together cover enormous areas of western and southwestern Montana, central and northern Idaho, and southeastern British Columbia. Lewis and Clark and their party surely saw thousands of outcrops of Belt formations as they trekked through the Northern Rocky Mountains.

The white sandstone and sand Clark saw above the argillite in the cliff are part of the Carmen formation, which geologist A. L. Anderson studied in 1956 and named after nearby Carmen Creek. It is a deposit of pale sediments and volcanic ash laid down on the land in early Tertiary time, or approximately 50 million years ago.

Of course Clark, like all the other men in the Corps, but especially the hunters, had to be a good judge of the kind of stone needed in a flintlock gun. Lewis had included "500 best Flints" in his pre-expedition shopping list, by which he probably meant imports from England. On May 18, 1803, he received 500 rifle flints and 125 musket flints from the Arsenal. He may have bought more in St. Louis, as the party grew from the original 15 men planned, to more than forty.1 By the time they got back to the east side of the Bitterroot Range in 1806, they must have been nearing the end of their supply. Clark's instructions to Sergeant Pryor, whose mission was to trail their horses from the middle Yellowstone River up to Fort Assiniboine in Canada to trade for services and supplies, included the purchase of gunflints as the highest priority.

Along the trail the men occasionally tested rocks with a piece of steel to see how "flinty" they were. Of the "confused and broken masses of stone" on the Salmon River's banks below the forks, Clark told Lewis (July 23, 1805) that they were "white or brown and towards the base of a grey colour and so hard that when struck with a steel, yeald fire like flint." Clark's closing phrase suggests that he realized it was not true flint. In fact, the rocks in the depths of the canyon are of the sedimentary and volcanic sort; they contain no flint. Although Indians sometimes resorted to argillite and quartzite for the making of arrow points, the results were consistently crude and less than satisfactory.

Actually, flint is a black or very dark brown variety of chert. It commonly occurs in many kinds of sedimentary rocks, but most notably in limestone, and chiefly in Europe. Chert is a microscopically crystalline variety of quartz with a waxy appearance. Given a sharp blow with another hard instrument such as the butt of an elk antler—a skill called knapping—it breaks along curving surfaces to produce razor-like edges. Some of the limestone formations that the Corps of Discovery crossed in western Montana contain round nodules of dark chert that they could have mistaken for genuine flint.

Lewis and Clark often used the word flint merely in a general sense, as they also used the terms granite, limestone, sandstone, and "freestone." For instance, in the abandoned parfleche Drouillard brought him from Shoshone Cove on the 22nd, Lewis found "some flint and the instrument of bone for manufactureing the flint into arrow points. some of this flint was as transparent as the common black glass and much of the same colour easily broken, and flaked of[f] much like glass leaving a very sharp edge." The latter would have been obsidian, though Lewis did not use the term. The rock that Indians used for cutting tools and arrow points was—depending on trade or local resources—obsidian,2 chalcedony, chert, quartzite, jasper, agate, or ignimbrite.3 All are hard, and all can be fractured into small pieces with sharp edges.43


1. Michael Carrick, personal communication, July 11, 2004. It is likely that more gun "flints" were purchased in St. Louis during the winter of 1803-1804. If so, they might have been local chert rather than imported gunflint.

2. Visitors to Yellowstone National Park may recall the 200-foot-high outcrop of obsidian 11 miles south of Mammoth Hot Springs. Archaeological evidence has indicated that cutting tools possibly mined there by the Tukadikas, the "sheepeater" band of Shoshones, were once traded at least as far east as Ohio. Obsidian results when lava cools faster than crystals can form.

3. Noah Webster, in his Compendious Dictionary of the English Language (Boston, 1806), defined flint simply as "a very hard kind of stone." Chert was "a kind of flint, flints lying in thin strata"; obsidian was "a silicious [sic] stone, black or grayish black"; jasper, "a beautiful white or green stone"; quartz, "a siliceous stone so hard as to emit sparks with steel"; agate, "a class of gems, of many varieties." Siliceous—from silex, the Latin word for flint— meant "pertaining to silex, or the genus of silex, flinty." None of the journalists ever used any of those terms, other than flint. Chalcedony and ignimbrite were not in Webster's first dictionary. Chalcedony is the category of quartz that includes jasper, agate, and onyx. The volcanic rock ignimbrite consists of consolidated fragments of pumice.

4. See also Discovery Path Geology, Inside Kirwan's Mineralogy.