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cleavage

(Encyclopedia) cleavage, tendency of many minerals to split along definite smooth planar surfaces determined by their crystal structure. The directions of these surfaces are related to weaknesses in…

Baltic Shield

(Encyclopedia) Baltic Shield, the continental core of Europe, composed of Precambrian crystalline rock, the oldest of Europe. The tectonically stable region was not affected by the Caledonian,…

silt

(Encyclopedia) silt, predominantly quartz mineral particles that are between sand size and clay size, i.e., between 1&fslsh;16 and 1&fslsh;256 mm (1&fslsh;406 –1&fslsh;6502 in.) in…

concretion

(Encyclopedia) concretion, mass or nodule of mineral matter, usually oval or nearly spherical in shape, and occurring in sedimentary rock. It is formed by the accumulation of mineral matter in the…

Columbia Plateau

(Encyclopedia) Columbia Plateau, physiographic region of North America, c.100,000 sq mi (259,000 sq km), NW United States, between the Rocky Mts. and the Cascade Range in Washington, Oregon, and…

gravel

(Encyclopedia) gravel, particles of rock, i.e., stones and pebbles, usually round in form and intermediate in size between sand grains and boulders. Gravel is composed of various kinds of rock, the…

slate

(Encyclopedia) slate, fine-grained rock formed when sedimentary rocks such as shale are metamorphosed by great pressure. Slate splits into perfectly cleaved, broad thin layers; this…

cuesta

(Encyclopedia) cuestacuestakwĕsˈtə [key], asymmetric ridge characterized by a short, steep escarpment on one side, and a long, gentle slope on the other. The steep side exposes the edge of erosion-…

Olympic Mountains

(Encyclopedia) Olympic Mountains, highest part of the Coast Ranges, on the Olympic Peninsula, NW Wash. Mt. Olympus (7,965 ft/2,427 m) is the highest point in the mountains, which are composed mainly…