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By five or six 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.