From Wikipedia: "The Grand Canyon of the Yellowstone is the first large canyon on the Yellowstone River downstream from Yellowstone Falls in Yellowstone National Park. The canyon is approximately 24 miles (39 km) long, between 800 and 1,200 ft (240 and 370 m) deep and from .25 to .75 mi (0.40 to 1.21 km) wide. The canyon below the Lower Yellowstone Falls was at one time the site of a geyser basin that was the result of rhyolite lava flows, extensive faulting, and heat beneath the surface (related to the hot spot). No one is sure exactly when the geyser basin was formed in the area, although it was probably present at the time of the last glaciation. The chemical and heat action of the geyser basin caused the rhyolite rock to become hydrothermally altered, making it very soft and brittle and more easily erodible (sometimes likened to baking a potato). The colors in the canyon are also a result of hydrothermal alteration. The rhyolite in the canyon contains a variety of different iron compounds. When the old geyser basin was active, the "cooking" of the rock caused chemical alterations in these iron compounds. Exposure to the elements caused the rocks to change colors. The rocks are oxidizing; in effect, the canyon is rusting. The colors indicate the presence or absence of water in the individual iron compounds. Most of the yellows in the canyon are the result of iron present in the rock rather than, as many people think, sulfur." Please zoom.