Goals
Introduce strike and dip and show how they can be used to specify the three-dimensional orientation of geologic features.
Demonstrate how geologic cross sections are drawn from maps.
Derive a set of rules for determining geologic structure from map outcrop patterns.
Illustrate how the subsurface orientation of beds can be recognized from map patterns.
   
Introduction
A geologic map is a graphic information display system. It illustrates the composition, structure, and history of geologic material or a portion of the Earth’s surface. Most important, a geologic map shows how geologic parameters change spatially within the area of interest.
Two very different types of information are displayed on a geologic map:
  • Descriptive information
    describes the physical features of an area in an objective and accurate manner. Features described might include bedrock geology; surface material; and landslides, sediments deposited by wind, water, or ice, and geologic structures. A geologist can directly observe these features in the field. A good geologic map will show where descriptive information is based directly on measurement and observation and where it is inferred. A general geologic map displays a wide variety of descriptive information, whereas a specialized geologic map focuses on only one or two types. For example, a volcanic hazards map will highlight recent volcanic rocks and their outcrop patterns. A map constructed to identify materials for road and railway construction will show only outcrops of unconsolidated surface materials.
  • Interpretative information
    explains how the geologic materials and structures of the region are related in time and space. These relationships are inferred from descriptive information and a geologist's understanding of  geologic processes. For example, the position and nature of an unconformity cannot be observed, measured, or otherwise directly quantified. Rather, its existence must be inferred from observations. Clearly, the quality of the interpretative information is only as good as the descriptive data on which it is built.

    Geologic maps are constructed on a variety of scales depending on the detail required. They may cover only a small local area or depict the geology of a continent. The choice of scale determines the detail with which the geology can be shown.
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    Fig. 1: U.S. Geologic Map of the continental United States.
     
    In the United States, the U.S. Geological Survey publishes numerous geologic maps annually - one of the primary responsibilities assigned to the Survey by the 1879 law that established it. Some state geologic surveys and universities also produce geologic maps. Maps drawn by private companies are typically not available to the general public.
    Geologic maps are used for a wide variety of purposes, including resource planning, energy and mineral exploration and production, evaluation of seismic and volcanic hazard risks, siting of nuclear power plants and waste repositories, identification of groundwater sources and pollution sites, and selection of transportation right of ways. The Departments of Energy, Defense, and Transportation, the National Forest and Park Services, and the Federal Emergency Management Agency use geologic maps regularly.In addition, most energy, mineral, and environmental firms routinely use this type of map.
      

     

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