A geologic map depicts the aerial distribution of various rock types of different ages.  Rocks are broken out into mappable units or formations.  A formation is a rock type that is distinct enough to separate from the other rocks in the area, and thick enough to show accurately on the map.  Each formation is represented by a unique symbol and color on the geologic map.  The symbol indicates the rock's age and formation name.  The geologic pattern and symbol for each formation are usually overprinted on a topographic map.  More . . .