A Story Written in Stone

A Story Written in Stone. . Plunging to a depth of a mile and a half and averaging 10 miles in width, Hells Canyon is the deepest gorge in North America. Its walls are an open book, revealing four significant chapters in the geologic history of the Pacific Northwest. , , Today’s landscape is just the most recent chapter. For over 6 million years, tremendous forces within the earth have been gradually uplifting the Wallowa and Seven Devils Mountains. Hells Canyon was carved during the last 2 million years, by a Snake River much more powerful that we know it today. , , But the story of this landscape is a long, complex one. Take a trip back in time some 300 million years to see where it began. , , Chapter One: An Island Arc Is Born , 300 to 130 Million Years Ago , , Between 300-220 million years ago, two oceanic plates in the South Pacific converged violently. Lava spewed from the ocean floor and the Blue Mountain Island Arc emerged. Then the island volcanoes lay dormant. From 210-130 million years ago, the island arc migrated, inch by inch, some 2000 miles northward on a moving plate on the earth’s crust. , , Chapter Two: Collision And Erosion , 130 to 17 Million Years Ago , , About 130-100 million years ago, the island arc collided with the ancient North American continent. When it “docked,” it forced up the mountains of central Idaho and displaced the Pacific Ocean to the west. The leading edge of the oceanic plate, was forced downward, as it slid under the continental plate. , , Chapter Three: Plateau Basalt , 17 to 6 Million Years Ago , , Lava flows poured from great cracks in the earth, burying the eroded landscape and filling in streambeds and other irregularities in the earth’s surface. Flow after flow blanketed the Hells Canyon region until a nearly level plateau was created. Known as the Columbia River Basalts, these lava flows covered about 70,000 square miles in Oregon, Washington and western Idaho. , , Chapter Four: Uplifting and Canyon Cutting , 6 Million Years Ago to Present