Hells Canyon, the deepest gorge in North America at a mile and a half deep and averaging ten miles in width, reveals four major chapters in the geologic history of the Pacific Northwest. Over more than six million years, forces within the earth gradually uplifted the Wallowa and Seven Devils Mountains, and during the last two million years a much more powerful Snake River carved the canyon. The story begins some three hundred million years ago, when between three hundred and two hundred twenty million years ago two oceanic plates in the South Pacific converged violently, lava erupted from the ocean floor, and the Blue Mountain Island Arc emerged. After the island volcanoes became dormant, the arc migrated about two thousand miles northward between two hundred ten and one hundred thirty million years ago on a moving plate in the earth’s crust. About one hundred thirty to one hundred million years ago, the island arc collided with the ancient North American continent, forcing up the mountains of central Idaho, pushing the Pacific Ocean westward, and driving the leading edge of the oceanic plate downward beneath the continental plate. From seventeen to six million years ago, lava flows from great cracks in the earth buried the eroded landscape, filled streambeds and other irregularities, and repeatedly blanketed the region with Columbia River Basalts until a nearly level plateau covered about seventy thousand square miles in Oregon, Washington, and western Idaho.