A geologic map shows the types of rocks or loose sediments at or below Earth's surface, along with their distribution. Geologic maps also illustrate the relative ages of, and physical relationships between, Earth's materials. Geologic maps are used for a variety of purposes, including natural resource development, land use planning, and natural hazard studies.

There are three steps to constructing a geologic map. First, the geologist locates natural or man-made exposures of rock called outcrops. Second, the geologist records outcrop locations and characteristics on a simple base map. Finally, the geologist prepares a geologic map by interpreting the distribution of and relationships between rock units.

Outcrops provide several kinds of data that are critical to map construction. The geologist records the rock type, for example, sandstone or granite, and a detailed description of its specific physical characteristics. Fossils, if present, may allow the rock to be...