Programs and Services

Geologic Mapping

A geologic map is a plane-surface graphical depiction of an area’s rock units and structures drawn using lines, symbols, patterns, and possibly colors. It commonly describes the composition of the rocks and correlates the units by position and time. It may also provide stratigraphic cross sections and tables of comparative data. A map represents the scientist’s blueprint of the extent, type, and ages of rocks and deposits and their arrangement on the surface and in the subsurface. This linear schematic interprets a moment in the evolving processes that have made our physical world.

The knowledge gained by revealing the Earth’s history has practical, beneficial applications for us. Geologic maps record information indispensable in our busy world. To summarize a few examples of their importance: They are instrumental in locating and evaluating metallic and nonmetallic minerals, hazards such as landslides and seismic faults, sources of potable water, sites for domestic and industrial waste, routes for streets and highways, and areas to direct urban expansion.

Creating an original geologic map involves, foremost, someone to "walk the beat," to become familiar with the area under study. Most veteran geologists agree there’s no substitute for being there, for doing their on-the-ground duty. Like an earnest detective, the geologist begins his or her investigation with the usual suspects but leaves no stone unturned in exposing the secrets of the Earth’s composition. Besides noting clues on the scene, the scientist will probably analyze a line-up of rocks, shift investigatory perspective through a birds-eye view of the area using aerial photographs, and pursue as necessary related geologic evidence. A geologic map’s accuracy and worth ultimately reflect the knowledge, skill, and thoroughness of the investigator.

The content of a map can be whatever the geologist chooses to emphasize and interpret. A map can be drawn to feature specific geologic conditions. A bedrock map, for instance, reveals the underlying rock structure and ignores surface units like alluvium. A surficial map, on the other hand, distinguishes only the soil and unconsolidated materials seen on the ground. Users of geologic maps—such as urban planners for example—may consult several types as they contend with commercial and residential construction, public utilities, local hazards like flood plains and earthquake faults, ground-water sources and recharge areas, the locations of industrial minerals for sand and gravel, and waste management systems. Geologic maps in all their variety and forms provide valuable information to guide decisions impacting the quality of our lives.

As for the level of detail in a map, that too is left to the interpreter. Certainly the scale of the mapping is the single most important determiner. The larger the scale, the more information that can be shown. Detailed geologic mapping refers mostly to work conducted on the 1:24,000-scale of a 7.5-minute topographic quadrangle. To get a broader perspective of an area, the geologist integrates the contiguous and overlapping work found in other maps and produces a reduced-scale map. This less-detailed, smaller scale compilation is meaningful for providing the big picture from which to view a region’s geology. With a geologic map, scientists can explore and share ideas about the Earth’s development on local, regional, and global scales.

The level of detail is also influenced by the intent of the mapping. Map titles that begin—"Geology of . . ." or "Geologic Map of . . ."—indicate a thorough interpretation of rock units. Terms that qualify a map title, like "reconnaissance" and "preliminary," indicate something less than definitive mapping. "Reconnaissance" refers to a cursory geologic exploration of an area, the minimum work necessary to address a project. The purpose might be for a quick overview to determine where detailed mapping should be undertaken. In this case, the geologist may be able to piece a reconnaissance map together in the office without going to the field. "Preliminary" identifies a product, though rough or incomplete, that contains original work and is being released to meet a deadline or answer a demand. These levels of detail are all equally valid approaches to producing geologic information for particular needs.