-----The Grand Canyon - 5/5 to 5/6/08

About 70 million years ago, the Rocky Mountains began to form, pushed up as the North American Plate overrode the Pacific Plate. In the process, a large section of what is now Utah, northern Arizona, western Colorado, and a corner of New Mexico rose from sea level to elevations of thousands of feet, forming the Colorado Plateau. This uplift occurred with remarkably little tilting or deformation of the sedimentary layers. The stage was set for the carving of the Grand Canyon.

By 5 or 6 million years ago, the Colorado River flowed across the Colorado Plateau on its way from the Rocky Mountains to the Gulf of California. Each rain washed sparsely-vegetated desert soils into the river. A steep gradient and heavy sediment loads created a powerful tool for erosion. The river's volume varied seasonally and over time. As the last Ice Age ended 12,000 years ago, the flow may have been 10 times today's volume.

As the river cuts down, the canyon deepens. Tributaries erode into the canyon's sides, increasing its width. Erosion carves faster into the softer rock layers, undermining harder layers above. With no foundation, these layers collapse, forming the cliffs and slopes profile of the canyon. Erosion wears away the ridges separating adjacent side canyons, leaving buttes and pinnacles.