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Science Tour - 3D/4D mapping of the San Andreas Fault Zone

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Map legend for USGS Map I-1705 providing description of rock units and geologic features on the map.

Example of a map legend: The geologic unit description is an essential component of the geologic map. Geologic map units describe assemblages of rock materials that are observable on a mapable scale (covering areas big enough to show larger than a dot on a map!). Rock units typically contain on type of rock of a known age and identifiable origin. However, in regions like northern California, earth materials of different ages and origin have been mixed, altered (metamorphosed through fault shearing pressure, geothermal heating, exposure to fluids (migrating groundwater, seawater, hydrocarbons, and other gases). Observing earth materials (rocks and soils) and interpreting them into meaningful, mapable units is part of the science of geologic mapping, and involves both exhaustive field work combined with laboratory testing (petrographic and geochemical analyses, fossil identification, and age dating methods using radioisotope geochronology). Over time, geologic units and their collective history must stand the test of scientific scrutiny and modified where necessary. New discoveries happen all the time that reshape the science of geologic mapping in a region. In addition, new landscape mapping methods using technologies such as LiDar, multispectral imaging, and other methods are changing the way map units are generated and used for interpretation.
Graphic provided by Robert McLaughlin; revised from:

McLaughlin, R. J., Ohlin, H. N., and Thormahlen, D. J., , 1990, Geologic map and structure sections of the Little Indian Valley - Wilbur Springs geothermal area, northern Coast Ranges, California: U.S. Geological Survey Miscellaneous Information Series Map, I-1706, scale: 1:24,000.
 

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