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 . . .