All geologic maps come with a table called a map key. In the map key, all the colors and symbols are shown and explained. The map key usually starts with a list showing the color and letter symbol of every geologic unit, starting with the youngest or most recently formed units (in the example map those are the man-made deposits), along with the name of the unit (if it has one) and a short description of the kinds of rocks in that unit and their age (in the key, the age is described by Epochs, subdivisions of the Periods shown in the letter symbol). After the list of geologic units, all the different types of lines on the map are explained, and then all the different strike and dip symbols. The map key will also include explanations of any other kinds of geologic symbols used on a map (locations where fossils were found, locations of deposits of precious metals, location of faults known to be active, and any other geologic feature that might be important in the area shown by the geologic map). Because the geology in every area is different, the map key is vital to understanding the geologic map.
Information provided by: http://geology.wr.usgs.gov