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When weathering and erosion expose part of a rock layer or formation, an outcrop appears. An outcrop is the exposed rock, so named because the exposed rock "crops out." Outcrops provide opportunities for field geologists to sample the local geology—photograph it, hold, touch, climb, hammer, map, sniff, lick, chew, and carry it home. Classes often visit outcrops to see illustrations of the principles of geology that were introduced in lecture. You often can see geologists or students identifying rocks in roadcuts, outcrops along the road where highway construction exposed the rocks.

Mountainous regions, where any loosened Earth material swiftly washes away, contain some of the best outcrops because a greater percentage of the rock formation lies exposed. Rocks crop out especially well across steep slopes, above the tree line (elevation above which trees cannot grow), and on land scraped free of soil by bulldozer-like glaciers. Sediment collects and plants grow in flatter areas, obscuring the rocks. In some areas soil and sediment may completely cover all the underlying rock, such as in the southeastern United States. However, in the desert southwest, the opposite is often the case. Outcrops cut the cost of mapping for geologists. The greater expense of geologic mapping in an outcrop-free area results from high-priced drilling to sample the rocks hidden below the surface.

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