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Why wouldn't rivers cut straight down through rock? When I use to hike in the north Georgia mountains I often ran across gorges cut by streams. Some of these gorges had shear vertical walls going up several meters. In some cases, the rock walls actually angled inward. The granite walls were not worn any smoother than the rocks I find in my back yard. I'm not sure the Grand Canyon really requires anything more than water erosion (and maybe the occasional sand blasting by wind) to account for its formation.