Importance of Geologic Maps
Geologic maps are probably the most definitive aspect of geology. They display the distribution of rock types and features and orientations of bedding, faults and other structural features on a standard map base. They are used extensively in mineral and ore exploration, petroleum exploration, and environmental analysis. These maps tell geologists where to drill for oil and gas or mine for gems, coal, strategic metals, and abrasives, or where pollutants might be migrating and of greatest concern. The ability to construct and interpret geologic maps is what sets geologists apart from all other scientists and engineers in these and other areas of study.

How is a Map Made?
Traditional geologic maps are constructed on a standard topographic base map by a geologist or team of geologists. The geologist visits a representative number of rock exposures in the area under study and records as much information as possible. This information includes the rock type and geologic unit to which it belongs, the orientation of the planar and linear features of the rock (bedding, cleavage, joints, faults, folds, mineral alignment), and the stratigraphic relations (contacts, lateral continuity, vertical changes). The geologist may take samples of the rock or measure geochemical or geophysical (gravity, magnetics, radioactivity, electrical, etc.) quantities as well. The location of the exposure can simply be determined by study of the topographic map or by using a Global Positioning System (GPS).

In the lab or at home, the information gathered in the field as well as that further elucidated by analytical work, a literature search of previous work and/or field data analysis is plotted on a topographic base map, point by point. Traditionally, this process involved picking representative colors for each unit and placing small swatches on the location of the exposures and applying the orientation data using a protractor and ruler. The map winds up looking like it has tutti-frutti measles. The final step involves establishing the contacts among units and connecting the dots to produce a map that is geologically reasonable. This last step can be harrowing and require weeks to months of positioning and repositioning until everything fits together as well as possible.

With the advent of personal computers and elaborate drafting programs, this process is now more commonly done digitally. Typically, the map is assembled in layers of topography, bedrock contacts, orientation data, surficial deposits and possibly even geochemical or geophysical data. The computer registers all of these layers perfectly. They can be assembled using very powerful Geographical Information Systems (GIS) programs yielding digital products that can be used for complex multivariate analysis (including ecological, epidemiological, and/or census data) and that can be universally accessed.

How are Geologic Maps Used?
Basically, geologic maps show distributions and arrangements of geological features. You can tell where certain rocks and features are located on a standard map. Although maps are completely covered with color, in reality, on average, only about 1% of the rocks in an area are exposed. The great majority of the bedrock is covered by soil and consequently not observable. Certainly, some areas have much greater exposure but only in certain regions. Some maps have special shading to show the exposures in an area but most don't. Exposures on most maps are marked by the structural symbols (strike and dip symbols) but only a representative set of these symbols are included on maps. Therefore the amount of rock that is visible in an area may not be determined.

The bands of color or pattern on the map represent a certain geologic unit. They are either single rock types or groups of closely related rock types. Contacts between units are marked with lines that are defined by the certainty of the location of the contact. Is it observed, estimated, covered? The succession of the color bands across the map reflects the stratigraphy. The pattern of the bands reflects the structure of the area. Folds form arcuate patterns and fold axes are indicated on the map. Faults are thick lines that truncate the color bands. A complete geologic history for an area may be interpreted by studying the map relations.