Fifteen years ago, my family visited Mesa Verde National Park, in southwest Colorado. Exploring the Sun Temple, we were suddenly startled by an explosion that echoed through the adjacent canyon and looked up to see a massive sheet of rock crashing down the canyon wall, bulldozing trees and everything in its path.
Initially carved by streams and glaciers, the valleys, canyons and gorges of our planet continue to erode through time. The primary stream and its tributaries relentlessly deepen the central chasm and its side canyons while the width of the gorge expands through other processes. Snowmelt and rain enter fissures in the rock walls and these cracks expand when the water freezes; over time, slabs of rock break from the wall of the gorge, tumbling into the valley below. These slump blocks are eventually broken down into smaller rocks by the same freeze-thaw cycle.
In other cases, rivulets of rain and snowmelt gradually erode soft layers of the canyon wall (shales, mudstones), producing recessed caves; when the loss of support reaches a critical point, the overlying rock layers collapse, sending massive boulders down the slope. Finally, in areas of poorly compacted sediment, prolonged or heavy rains saturate the outer layers of the gorge wall, which eventually pour into the valley as an avalanche of rock and mud. Over millions of years, these processes blend the canyon with the surrounding uplands, producing a rolling landscape with less dramatic relief; of course, this transition is much slower in arid regions.
