Rocks of Ages:
An explanation of the legend
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Click on a section of the legend to learn more about that part of geologic time! |
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Geologists subdivide time variously by eras (Precambrian to Cenozoic), periods (Cambrian to Quaternary), and epochs - only Pleistocene (early Quaternary) and Holocene (late Quaternary) are used here (see column at left). These units are quite uneven in elapsed time, the older intervals generally being of much longer duration. Rocks in the U.S. range in age from early Precambrian (2.6 billion years ago) to Holocene, which includes the present. The orderly sequence of Earth materials, from oldest to youngest, is represented by an equally well-ordered sequence of "prismatic" colors (based on the rainbow). To improve the color balance of the tapestry, the Holocene is represented by two hues, light gray in the East and beige in the West.
The King and Beikman geologic map of the U.S. was compiled from many detailed maps that describe the rocks of smaller areas. The constituent maps were made by many individuals from field and laboratory observations. These geologic maps distinguish among types of rocks that form in different ways: igneous granite and basalt; sedimentary sandstone, shale, and limestone; and metamorphic slate, marble, gneiss, and schist. From several converging lines of evidence - fossils, the layered sequence of strata, and the systematic radioactive decay of certain minerals - geologists have been able to place the rock formations in their correct time order, and from that arrangement construct a sequence of likely geologic events. |
