[Illustration: THE ROCKS of Yellowstone National Park, separated into individual units or formations and arranged according to their geologic ages (see fig. 6). A formation is a body of rock that contains certain identifying features (such as composition, color, and fossils) which set it apart from all other rock units. The identifying features of each formation provide valuable clues bearing on its origin. Most formations are given formal names, and usually each formation is thick and widespread enough to be recognized over broad areas. Some, however, change character from place to place, and different names may be used in different areas even though the rocks represent the same geologic time interval. (Fig. 5)] AGE, IN THOUSANDS OF YEARS ROCK FORMATION OR UNIT 40± to present Stream sand and gravel Hot-spring deposits 9 to 250± Glacial deposits 60 to 600 Plateau Rhyolite 600 Upper Unit, Yellowstone Tuff 600 to 2,000 Rhyolite and basalt lava flows 2,000 Lower Unit, Yellowstone Tuff 2,000+ Rhyolite and basalt lava flows KINDS OF ROCKS SHOWN IN COLUMNS Sandstone or stream sand Conglomerate, glacial moraines, or stream gravels Volcanic breccia Shale Limestone Dolomite Lava flows Welded tuff Travertine or geyserite ROCK FORMATIONS AGE, IN PERIOD ERA MILLIONS OF YEARS Northern part of Southern part of park park