The most striking features of geologic maps are its colors. Each color represents a different geologic unit. A geologic unit is a volume of a certain kind of rock of a given age range. So a sandstone of one age might be colored bright orange, while a sandstone of a different age might be colored pale brown. Many geologic units are given names that relate to where their characteristics are best displayed, or where they were first studied. For example, the Briones sandstone was first described in Briones Valley,California.

Some geologic units have not yet been named, so those are identified with terms related to the kind of rock in the unit like 'Sandstone and shale,' 'Unnamed sandstone', or 'Undivided shale'. But all units, named and unnamed, have a color on the geologic map, and the area of a given color is the area where that geologic unit is the one at the surface (usually the soil on top of the rocks is disregarded).

Geologic units are named and defined by the geologists who made the geologic map, based on their observations of the kinds of rocks and their investigations of the age of the rocks. As more information is gathered, perhaps by other geologists, new geologic units might be defined. These disagreements can be a basis for scientific progress, and illustrate the need for continuing to investigate the geology of an area.

Information provided by: http://geology.wr.usgs.gov