At Mammoth Hot Springs, beautiful terraces are formed by gently flowing waters. The waters deposit a form of limestone called travertine, building large terraces one above the other. Algae and bacteria give some terraces various colors. Minerva Terrace and Opal Terrace are among the most beautiful in the area. The terraces change through the years as the waters deposit calcium carbonate, a mineral that builds up the formations.
Some springs die, and the terraces become gray and lifeless.
The Hoodoos are the remains of old hot-spring terraces broken up by landslides. Obsidian Cliff is a mountain of black glass that was formed by molten lava. Rootless vegetation called lichens now cover the glass in many places. Minerva Terrace Norris Geyser Basin consists of hundreds of geysers, hot springs, and pools. It is the hottest and most active thermal area in Yellowstone.