In geologic time, the Teton Range is a young and active part of the Rocky Mountain chain, still rising even as erosion continually reshapes it. Nine million years ago, the earth's crust broke into two rectangular blocks along the Teton fault, a 40-mile-long zone of weakness. Through sporadic movements, the western block hinged upward to become the Teton Range, while the eastern block tilted downward to form the valley called Jackson Hole, and the valley sank four times more than the mountains rose. Displacement continues, and an earthquake-producing movement along the fault can occur at any time. Wind, water, ice, and glaciers, especially during the last Ice Age, shaped the range into today's skyline, and although the sedimentary rock layers that once covered the central peaks have been worn away, mountain building continues to counteract erosion.