File:Grand Canyon of the Yellowstone River (Yellowstone, Wyoming, USA) 270.jpg

English: This spectacular and colorful valley in Yellowstone, Wyoming is the Grand Canyon of the Yellowstone River. Two large waterfalls occur along this stretch of the river - Upper Falls and Lower Falls. The rocks of the canyon include rhyolite lava flows, rhyolitic volcanic tuffs, and some sedimentary deposits - all are geologically young and date to the Quaternary. The yellows, pinks, reds, and oranges indicate the presence of hydrothermally altered rocks. Hydrothermal metamorphism refers to the intense alteration by superheated groundwater. Yellowstone has over 100,000 hydrothermal features in the form of hot springs, geysers, fumaroles, frying pans, and mudpots, so hydrothermal alteration is fairly common in the area.

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From park signage:

The Force of Water

This canyon has been a work in progress for at least the last 12,000 years. After the last caldera forming eruption 640,000 years ago, this area was covered by a series of lava flows. Rising hot magma caused the earth to dome and erosion to occur at a faster rate. This heat also drove superheated water through the hardened lava, altering it to the reds and yellows around you. As the river flowed through this softer rock, a V-shape was eventually carved, forming a canyon. This process may have had a little help though. It is possible that ice dams formed upriver near the end of the last glacial period. When these dams melted or broke, a giant torrent of water would have rushed downstream, resulting in the immediate and catastrophic erosion of the present canyon. This may have happened more than once. The Yellowstone River continues the process in a slower manner today.

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Locality: view from the Inspiration Point area, northern rim of the Grand Canyon of the Yellowstone River, northwestern Wyoming, USA