Following an Indian road on the east side of the Beaverhead River, Captain Meriwether Lewis and three men arrived here on August 10, 1805, paused to eat freshly killed deer, and named the high rock formations "rattle snake clifts" because of the many snakes they encountered. Lewis had gone ahead of the main party several days earlier to search for Sacagawea's people, the Lemhi Shoshoni. Five days later, on August 15, 1805, Captain William Clark came through with the main party, pulling heavy canoes against the river current, and also encountered many rattlesnakes. These cliffs rise about 600 feet above the valley floor and are formed of 50 million-year-old volcanic rock uplifted along the Blacktail fault; weathering in the fractured rock created many small hollows that make excellent snake dens. That day Clark narrowly escaped being bitten while on shore, Sacagawea also narrowly escaped, and Joseph Whitehouse recorded that some of the party caught fine trout while Clark killed a rattlesnake that got between his legs as he stood fishing, along with a number of others that day.