CGS's Regional Geologic Mapping Project maps and compiles the geology of the state at various scales with the objective of developing a statewide geologic map database. Collaboration with the U.S. Geological Survey under STATEMAP and SCAMP augments this project.

Caltrans Highway Corridor Mapping, began in 2000, maps geology, existing landslides and potential slope instabilities along the major Interstate highway corridors that slice through California’s mountainous areas.

The Mineral Resources and Mineral Hazards Mapping Program produces geologic maps of areas being classified for mineral resources (mineral land classification), and of areas where naturally occurring mineral hazards (asbestos, mercury, and radon) are more likely to occur

The Timber Harvesting Plan Enforcement project provides engineering geologic review of Timber Harvesting Plans (THP), watershed assessments, and other regional scale documents.

The Seismic Hazards Zonation Program uses geologic maps to help account for the effect earth materials have on damaging ground shaking and ground failure during an earthquake. Earth materials are regrouped and classified according to their adverse effects on buildings and other man-made structures.

• What is a Geologic Map?

• Use of Geologic Maps

• Value of Geologic Maps

• CGS Programs that Produce Geologic Maps

• Related Links About Geologic Maps

• View and download Generalized Geologic Map of California (PDF file, requires Adobe Acrobat or Acrobat Reader) Page-size PDF (241 KB)   Full-size PDF (866 KB)     

Portion of the "Geologic Map of Monterey 30'x60' Quadrangle and Adjacent Areas, California".
California Geological Survey CD 2002-04
-Click on image for a larger view-

What is a Geologic Map?

A geologic map shows the distribution, relationship, and composition of earth materials such as rocks and surficial deposits (landslides, sediments) and shows structural features of the earth (faults, folded strata). Special purpose maps or derivative maps may only show details of faulting, landsliding, mineralization or other geologic processes or features.   A geologist compiles the data to be presented on a map through the use of field observations, aerial photographs, remote sensing information, topographic information, soil survey information, and laboratory analyses. The geologic data, in layer fashion, are typically printed upon a base map. The base map may show the topography,  roads, rivers, or other cultural or natural features in the map area. Modern maps are in digital form, extending their usefulness in decision-making by allowing geologic information to be combined with other data, such as demographics or infrastructure. Explanatory text on the maps provides information on the geologic age of the materials as well as their physical properties.

 A segment of a typical geologic map is shown above. On the map the geologist has recorded the locations, types, and ages of the rock units and surficial materials by using  various standard colors, symbols and patterns overlain on a topographic base map. The basic rock units shown on the map are called formations. For example on the map the green area labeled "Kp" shows the areal extent of the Panoche Formation of Cretaceous age. Similarly, the portion of the map labeled "Mvqa" represents in map view the areal extent of an unnamed formation composed of a volcanic rock called andesite of Miocene age.  A formation is usually named after a geographic feature (mountain, canyon, or town) near the area where the unit was first identified. The geologically most recent deposits on the map are the landslides ("Qls") depicted in yellow with arrows showing the direction of movement or "flow". Using special symbols, the geologist has recorded other significant observations on the map such as faults (bold black lines), folds, contacts between rock units, and the strike and dip of formations (respectively, the direction of a horizontal line within a unit and the angle that the unit slopes in outcrop). The explanatory text for the geologic map above is not shown. For more information about geologic maps see the "Related Links About Geologic Maps" below.

Use of Geologic Maps

A geologic map is the principal tool of a geologist or engineer who either needs to convey or decipher information about the Earth’s surface. Geologic maps are used to interpret the structure, stratigraphy, mineralogy, paleontology, and the historical record of the Earth’s crust. Geologic maps are used to locate energy resources (such as petroleum, natural gas, coal, and geothermal resources). They are used to locate sources of groundwater and mineral deposits (such as gold, iron, copper, clay and construction aggregate). Geologic maps are also used to identify areas that  may contain potential mineral hazards, such as mercury, asbestos, or radon. Geologic maps are used to identify potential geologic hazards such as landslides, earthquake fault zones, areas susceptible to liquifaction,  historic flood zones, volcanoes and areas susceptible to tsunamis. Geologic maps are used by land-use planners to identify and determine which areas are suitable for agriculture and urban development. They are a fundamental resource for environmental and engineering applications. Geologic maps provide an enormous amount of information needed to understand the Earth and to understand the geology of other planets by comparison. 

Value of Geologic Maps

The value of geologic maps is illustrated by the direct economic gain from the discoveries of energy resource deposits (coal, natural gas, oil, or geothermal sources), industrial mineral resource deposits (iron-ore, copper, construction aggregate, limestone, borates, or rare-earth deposits), and precious mineral resource deposits (gold, silver, or gemstones). Indirect socioeconomic value is gained through the use of geologic maps for making wise land-use planning and safe engineering decisions. Such decisions are based on avoidance of natural hazards, optimizing site conditions for construction projects, and conserving natural resources.

For Additional  Information About Geologic Maps See These Related Links:

• U.S. Geological Survey - "Geologic Maps"
  (http://www2.nature.nps.gov/grd/usgsnps/gmap/gmap1.html)

• U.S. Geological Survey - "What is a Geologic Map?"
  (http://geology.wr.usgs.gov/wgmt/aboutmaps.html)

• U.S. Geological Survey - "Maps and Images - Geologic Maps"
  (http://interactive2.usgs.gov/learningweb/explorer/topic_map2.htm)

• Utah Geological Survey - "Geologic Maps - What Are You Standing On?"
  (http://geology.utah.gov/online/pi-66/index.htm)

• Louisiana Geological Survey - "The Value Of Geological Maps"
  (http://www.lgs.lsu.edu/map.htm)