USGS Logo Geological Survey Bulletin 1191
Black Canyon of the Gunnison: Today and Yesterday


Many terms in day-to-day professional use by geologists are unfamiliar or vaguely familiar to nongeologists. Although the attempt has been made to minimize such usage in this report, some semitechnical jargon is inevitable in any discussion of technical matters. The following short glossary is not intended to be definitive, but it should help clarify most of the terms that are apt to be stumbling blocks. Many of the definitions have been borrowed, with slight modification, from a "Glossary of Selected Geologic Terms" by W. L. Stokes and D. J. Varnes, published in 1955 by the Colorado Scientific Society, Denver, Colo.

Amphibolite, a metamorphic rock of medium to coarse grain composed chiefly of a mineral of the amphibole group (generally hornblende) and plagioclase feldspar.

Anticline, a convex-upward fold; that is, one in which the limbs or sides slope away from the crest, like an inverted trough.

Ash flow, a hot turbulent mixture of gas and volcanic dust erupted down the slope of a volcano; on coming to rest, commonly compacts into a hard coherent rock. See also "Welded tuff."

Badland, an area of intricate erosional dissection, nearly free of vegetation and characterized by steep slopes, sharp ridge lines, and a maze of ravines or gullies. Badlands are abundant in Mancos Shale terranes west of the Black Canyon.

Bedding, the arrangement of rocks, especially sedimentary rocks, in layers or strata.

Breccia, a consolidated rock composed of angular rock fragments. Volcanic breccias are abundant in the West Elk and San Juan Mountains.

Competence, as applied to streams—capacity to transport debris; as applied to rocks—resistance to deformation or capacity to transmit stress.

Conglomerate, a rock, the consolidated equivalent of gravel.

Crossbedding, a diagonal arrangement of layering in sedimentary rock in which layers are inclined at various angles to more general planes of stratification. In the Black Canyon area, common in sandstones of the Junction Creek, Morrison, Burro Canyon, and Dakota Formations.

Desert varnish, a dark-colored surface stain or crust of iron or manganese oxide, coating exposed rock surfaces in desert or semiarid regions.

Diabase, a dense dark-colored igneous rock composed essentially of augite and plagioclase. Forms large dikes in Black Canyon area.

Dike, a sheetlike body of igneous rock intruded while molten into fissures in other rocks. Very abundant in the Black Canyon.

Dip, in structural geology, the angle between a sloping plane, such as a bedding plane or a fault, and an imaginary horizontal plane.

Eolian, pertaining to the wind.

Fault, a fracture in the earth's crust along which there has been movement parallel to the fracture plane.

Fault block, a mass of rock bounded laterally by faults.

Fold, a bend or flexure in layered rock. See also "Anticline," "Monocline," and "Syncline."

Foliation, layering or lamination in metamorphic and igneous rocks.

Gneiss, a layered metamorphic rock having alternate layers of visibly unlike mineral composition, especially, feldspar-rich layers alternating with mica-rich layers.

Gradient, as applied to streams, the inclination of the bed.

Granite, in a strict sense, a visibly granular igneous rock of interlocking texture composed essentially of alkalic feldspar and quartz, commonly with a small percentage of mica or hornblende.

Igneous rock, a rock formed by solidification of hot molten material, either at depth in the earth's crust or erupted at the earth's surface.

Joint, a fracture along which there has been little or no movement parallel with the fracture plane.

Laccolith, a lens- or dome-shaped mass of igneous rock injected between strata so as to distinctly updome its roof.

Laramide orogeny, the mountain-forming episode in which the Rocky Mountains were uplifted.

Limestone, a sedimentary rock composed chiefly of calcium carbonate.

Mass wastage, any process by which large masses of earth material are moved downward by gravity.

Metamorphic rock, rock changed materially in composition or appearance by heat, pressure, or infiltrations at some depth in the earth's crust below the surface zone of weathering.

Mineral, a naturally occurring substance of definite chemical composition and distinctive physical and molecular properties.

Monocline, a steplike bend in otherwise horizontal or gently dipping strata.

Orogeny, the process or the episode in which mountain ranges are formed.

Pegmatite, a name applied to various crystalline igneous rocks characterized by large average grain size and interlocking texture. Very abundant in Black Canyon area.

Peneplain, an extensive land area of very low relief produced by subaerial erosion.

Petrography, a branch of geology that deals with the description and classification of rocks; applied especially to microscopic study of rocks.

Phenocryst, a crystal in an igneous rock, embedded in a finer grained groundmass.

Physiography, as commonly used, the study of the surface features of the land. Also called physical geography.

Pluton, a general term applied to any body of intrusive igneous rock, regardless of shape or size, but commonly applied to bodies of deep-seated origin.

Porphyry, an igneous rock containing abundant phenocrysts set in a finer grained groundmass.

Quartz monzonite, a granitelike igneous rock containing more than 5 percent quartz and nearly equal amounts of alkalic and soda-lime (plagioclase) feldspars. A prevalent rock type in the Black Canyon.

Ripple mark, undulatory washboardlike markings produced on sand by waves, currents, or winds and commonly preserved in consolidated sandstone. Abundant in the Dakota Sandstone of the Black Canyon area.

Rock, any naturally formed aggregate of minerals constituting a sizable part of the earth's crust. Most rocks contain two or more mineral varieties.

Sandstone, a rock composed of cemented sand grains.

Schist, a crystalline metamorphic rock composed chiefly of platy mineral grains such as mica, oriented so that the rock tends to split into layers or slabs. Common in the Black Canyon.

Sedimentary rock, rock formed by consolidation of sediment deposited at the surface of the earth through action of water, wind, glaciers, or organisms.

Shale, a fine-grained sedimentary rock formed from muds or clays and having a fissility that causes it to split along planes parallel to the bedding.

Stock, an irregular conical or cylindrical body of intrusive rock which cuts across the enclosing rock and has a horizontal section of not more than 40 square miles. Larger bodies are called batholiths.

Superposition, as applied to drainage, a drainage pattern impressed on a formerly existing geologic terrane and let down by erosion onto different rocks and geologic structures, often with anomalous-seeming results.

Syncline, in simple form, a concave-upward, or trough-like, fold.

Talus, an accumulation of coarse angular rock fragments derived from and resting at the base of a cliff or very steep slope. Very common in the bottom of the Black Canyon.

Tuff, a volcanic rock formed by induration of volcanic ash or dust. Abundant in the West Elk and San Juan Mountains.

Unconformity, an erosion surface separating an older from a younger sequence of rocks. Formed when strata or volcanic rocks are deposited on an old land surface. An excellent example is preserved at the rims of the Black Canyon.

Welded tuff, a volcanic rock in which the ash fragments have retained sufficient heat following eruption to become partly or completely welded together after emplacement. Excellent examples in the caprocks of the mesas near the head of the Black Canyon.

Stellars jay

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Last Updated: 28-Mar-2006