The Grand Canyon of the Yellowstone is 20 miles long, up to 4,000 feet wide, and 1,200 feet deep, and its walls are the color of a sunset held still. The distinctive yellows, oranges, pinks, and reds come from hydrothermal activity: for thousands of years, heated water percolating through the rhyolite has altered the chemistry of the rock, oxidizing iron compounds into the spectrum of colors visible from every overlook along the South Rim Drive.
The Lower Falls of the Yellowstone River anchor the canyon's upper end. At 308 feet, they're higher than Niagara Falls and the most photographed feature in the park after Old Faithful. From Artist Point on the South Rim, the view is the canonical one: the falls dropping into the canyon, the river threading between walls that change color as they descend. Early morning is best — the light comes in level and warm and the colors in the rock face are most saturated.
The Yellowstone River drops an average of 70 feet per mile through this canyon, cutting through rock that hydrothermal alteration has weakened and made erosible. The river is doing now what it's been doing for two million years: carving deeper. The canyon is not finished.
The North Rim Trail gives you different perspectives from the overlooks at Inspiration Point and Grandview Point, both worth the short walks. The canyon from any of these vantage points is one of the more genuinely surreal landscapes in a park full of surreal landscapes. Yellowstone has geysers and hot springs and bison, but the canyon is what I keep returning to in memory.