Geologic Map

Geologic maps are graphical representations of rocks, sediments, and other geologic features observed or inferred to exist at or beneath Earth's surface. They can be based on observations of outcrops in the field, interpretation of aerial photographs or satellite images, and information obtained during the drilling of exploratory boreholes. Outcrops can be obscured, particularly in areas covered by dense vegetation or thick soil, and borehole information is often limited. Therefore, geologic maps are in most cases interpretive rather than purely descriptive scientific documents. Geologic maps are used for a variety of purposes, including petroleum, mineral, and groundwater exploration; land use planning; and natural hazard studies.

The first modern geologic maps were drawn by William Smith (1769-1839), a British canal builder. He recognized that sedimentary rocks occurred in a consistent sequence throughout the countryside. Knowing the position of a coal bed within the sedimentary rock sequence in one location allowed Smith to predict its occurrence and depth beneath the surface in other locations. Likewise, knowledge of rock sequences allowed Smith to predict the kinds of rocks that would be encountered during canal construction.

A general geologic map classifies rocks primarily according to their ages and secondarily according to their formation names. Formations are rock units that have a distinctive appearance or physical properties that can be identified in the field, and must also be laterally extensive and thick enough to depict on maps of a specified scale. In the United States, the scale at which formations must be mappable is 1:24,000, which corresponds to the scale of U.S. Geological Survey topographic maps covering 7.5 minutes of latitude and longitude.

Formations are named according to a protocol that requires a published description of each proposed formation in a scientific journal. If a formation consists of only one rock type, that rock type is included in the formation name (Berea Sandstone). If a formation consists of different rock types, then the word Formation is appended (Morrison Formation). Formations are named after nearby landmarks and, unlike fossils, never named directly after people. They can be, however, named after places that are named after people. The Gene Autry Shale, for example, is named after the town of Gene Autry, Oklahoma. The town, in turn, was named after the famous American singing cowboy Gene Autry (1907-1998).

Geologic maps also contain symbols representing geometric elements that are collectively known as geologic structures. These include faults, joints (fractures across which very little movement has occurred), aligned prismatic or platy mineral crystals known as lineations or foliations, and strata that have been tilted or folded in response to stresses within Earth's crust.

A simplified geologic map. Photo Researchers, Inc. Reproduced by permission.

Specialized geologic maps can also be drawn. Engineering geologists conducting investigations for construction projects, for example, may be more concerned with rock types than ages. They may therefore depict rocks using a system that emphasizes rock type and origin over age. Petroleum geologists and hydrogeologists often prepare maps based solely on information obtained during the drilling of oil, gas, and water wells. Those that depict changes in the thickness of a particular formation are known as isopach maps. Structure maps show the elevation or depth of a formation that may be an important petroleum reservoir or aquifer.