When Meriwether Lewis reached the "two forks," he marveled that the Missouri and Jefferson's rivers could run so far through such a mountainous country and remain so navigable. That landscape had been shaped over millions of years as the Earth's crust in this area was pulled apart, creating towering mountains and broad valleys. The calm rivers that followed those valleys made a Northwest Passage seem possible, but west of the Great Divide the geology formed deep canyons and raging rivers that ended hopes for a safe communication across the continent by water. Red Rock Valley began forming about 4.0 million years ago when extension of the Earth's crust produced a fault that dropped the valley and uplifted the adjacent Tendoy Mountains. Although geologists consider this fault active, it last ruptured the ground surface about 3,000 years ago, and there was no activity on August 17, 1805, when a historic meeting of cultures took place in the valley.