Charting the Rocks Below

To anyone unfamiliar with them, geological maps can look like the work of a child who went crazy with Magic Markers, using every possible hue in seemingly random patterns. This is particularly true of "A Tapestry of Time and Terrain," an online map created by the U.S. Geological Survey (USGS) at tapestry.usgs.gov. It combines vivid colors assigned to rocks of different time periods with topographic relief shading to illustrate some of the major geological features of the continental United States.

By clicking on "Rocks of Ages" on the main page, you can check the color-coded geological time scale to see how old the rocks are in your area. You can also select a slice of time--say, the Cretaceous--and a map will display the distribution of rocks from that period only. Click on "Description of Features" and you can zoom in on one of forty-eight points on the map to get a closer look and a brief description of the local geology.

Number twenty-six, for example, shows the Los Angeles area and describes how the relative westward movement of Earth's crust north of the Garlock Fault interferes with the movement of tectonic plates along the San Andreas Fault--a wrench in the works that threw up the mountains upon which my house rides. The map gives a nice tour of the nation's geology, but it left me wanting to zoom in much closer and to examine more than the forty-eight points offered.

If this Web site piques your curiosity and you want to learn more about the rocks that underlie your locality, the USGS also offers the National Geologic Map Database--a searchable catalog of both paper and digital maps (ngmdb.usgs .gov/). You can select from different kinds of maps and a host of other resources. Andrew Alden, About.com's geology guide, has also gathered geological maps of most states at geology, about .com/education/geology/msub24.htm.

Robert Anderson is a freelance science writer based in Los Angeles.

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