San Francisco Bay Region Geology and Geologic Hazards
Stories > What is a Geologic Map?
What is a Geologic Map?
A geologic map shows the distribution of geologic materials and geologic structures that are visible at the Earth's surface.
Geologic materials are the igneous, metamorphic, and sedimentary rocks and surfical sediments that form the landscape all around us. Most geologic maps, like this one, use colors and labels to show areas of different geologic materials, called map units. Geologic structures are the breaks and bends in the geologic materials and are caused by the slow but powerful forces that shape our world. Geologic maps show the location of these structures with different types of lines. Because the Earth is complex, no two maps show the same materials and structures, and so the meaning of the colors, labels, and lines is explained on each map.
This geologic map was made by bringing together and simplifying many separate geologic maps prepared by U.S. Geological Survey, California Geological Survey, and consulting geologists over the past 15 years. You may note some minor discontinuities in the map, mostly in remote areas. These are places where geologists have disagreed on the identity of the geologic materials, and they mark areas where more work needs to be done. All geologic maps can be made even better by further studies.