The Yellowstone River cut down 800 feet to form the Grand Canyon of the Yellowstone. The bright yellow to red rocks of the canyon walls are made of 630,000-year-old solidified lava highly altered by exposure to chemicals, hot water, and steam. Erosion was accentuated by glaciers along with increased water flow as the ice melted. The 310-foot-tall Lower Falls formed where resistant lavas overlie less resistant rock.Photo courtesy: Robert B. Smith

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