3.3: Geological Maps
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Geologic maps are two dimensional (2D) representations of geologic formations and structures at the Earth’s surface, including formations, faults, folds, inclined strata, and rock types. Formations are recognizable rock units. Geologists use geologic maps to represent where geologic formations, faults, folds, and inclined rock units are. Geologic formations are recognizable, mappable rock units. Each formation on the map is indicated by a color and a label. For examples of geologic maps, see the Utah Geological Survey (UGS) geologic map viewer.
Formation labels include symbols that follow a specific protocol. The first one or more letters are uppercase and represent the geologic time period of the formation. More than one uppercase letter indicates the formation is associated with multiple time periods. The following lowercase letters represent the formation name, abbreviated rock description, or both.
Cross-sections are subsurface interpretations made from surface and subsurface measurements. Maps display geology in the horizontal plane while cross-sections show subsurface geology in the vertical plane. For more information on cross-sections, check out the AAPG wiki.
Strike and Dip
Geologists use a special symbol called strike and dip to represent inclined beds. Strike and dip map symbols look like the capital letter T, with a short trunk and extra-wide top line. The short trunk represents the dip and the top line represents the strike. A Dip is an angle that a bed plunges into the Earth from the horizontal. A number next to the symbol represents a dip angle. One way to visualize the strike is to think about a line made by standing water on the inclined layer. That line is horizontal and lies on a compass direction that has some angle with respect to true north or south (see figure). The strike angle is that angle measured by a special compass. E.g., N 30° E (read north 30 degrees east) means the horizontal line points northeast at an angle of 30° from true north. The strike and dip symbol is drawn on the map at the strike angle with respect to true north on the map. The dip of the inclined layer represents the angle down to the layer from horizontal, in the figure 45o SE (read dipping 45 degrees to the SE). The direction of dip would be the direction a ball would roll if set on the layer and released. A horizontal rock bed has a dip of 0° and a vertical bed has a dip of 90°. Strike and dip considered together are called rock attitude.
This video illustrates geologic structures and associated map symbols.