Photograph showing the relationship between the landscape and several map unit symbols. Each of these map units provides important clues to the geologic history of the region, and to nearby geologic resources and geologic hazards. The photograph shows the Virgin River (at extreme lower right) and associated floodplain sediments (Qal1). Here, the Virgin River has eroded into the Kayenta Formation (Jk), which reveals rock layers tilted to the left. The Kaytena beds are overlain by a remnant of a much younger, nearly horizontal lava flow (Qb). Old river gravels (Qal3) now form a high terrace to the right, and talus (Qmt, angular blocks that tumbled downhill) covers much of the steep slope below the lava flow.

As for all those bright map colors and strange symbols, well, think of them as geologic shorthand. They may appear cryptic, but each geologic map is accompanied by a chart that explains what each line, color, and symbol means. Each map color, for example, represents a unique map unit - a group of rocks or sediments sufficiently distinct in age, composition, or environment of deposition so as to be differentiated from all other map units. By tracing map units across the countryside, geologists can locate earthquake faults, folds, and other geologic structures.

Each map unit is also identified by a unique symbol. The first letter of these symbols is capitalized and denotes the age of the unit, while subsequent lower-case letters identify the name of the map unit. The symbol TRmu, for example, is shorthand for the Triassic-age (TR) upper red member (u) of the Moenkopi Formation (m), a sequence of red mudstone and fine-grained sandstone deposited in a tidal-flat environment about 240 million years ago. Other lines and symbols on a geologic map provide information on geologic structures such as faults (normal and thrust), folds (anticlines and synclines), and the orientation (strike and dip) of rock layers, among other things.