Where the Beaverhead River entered the Blacktail Fault, the landscape changed dramatically. This fault uplifts the Blacktail Mountains more than 9,000 feet, and over the last six million years it created topography and remains active. As the Blacktail Mountains were uplifted, the Beaverhead River was forced to cut into hard volcanic rock, forming Beaverhead River Canyon. The river still flows in those rocks and continues cutting the canyon as the mountains continue to rise. Meriwether Lewis wrote on August 10, 1805, of halting on the river under an immensely high perpendicular cliff of rock, cooking and eating a hasty meal, and departing. In 1805 the Corps of Discovery passed quickly through the “rattle snake clifs,” focused on finding the Lemhi Shoshone for horses and information about the trail ahead. Here the north-flowing Beaverhead River crosses the Blacktail Fault and abruptly leaves the confines of a canyon for the broad expanse of the Beaverhead Valley.