World of Earth Science | Geologic Map

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 dated fairly accurately. If more than one rock type is present, the nature of the contact between them is important. In addition, the geologist records the shape and spatial orientation of each rock unit.

The accuracy of a geologic map is primarily dependent upon the spacing of outcrops. If outcrops are widely spaced, the geologist must use his or her knowledge of local and regional geology to fill in, or interpolate, between the outcrops. In most cases, the best interpretation will be the simplest one that fits all the known data.

The spacing of outcrops is dependent upon several factors. If the terrain is steep or vegetation is sparse, outcrops are better exposed. Rock units that are resistant to erosion also form good outcrops. Finally, in flat terrain, resistant beds are better exposed if they are steeply inclined (due to deformation).

The fundamental rock unit for mapping is the formation. A formation is a body of rock consisting of one or more rock types, usually at least 10 ft (3 m) in thickness, that is present over a large geographic area. These characteristics allow it to be the basis for geologic mapping. Formations are named for a geographic location, such as a town, near where

they were first described. For example, the Miami Limestone was named for Miami, Florida.

The distribution of each formation is shown on the map using a separate color, a letter code, or both. A line indicates where the contact is located between adjacent formations. If the contact is exposed at the surface, a solid line is used; if covered, a dashed line is used. Heavier lines indicate the location of faults. With training, one can learn to determine the geologic history of an area from looking at a geologic map.