  Alluvial fan. A cone-shaped mass of sediment built by rivers or streams as they issue from mountains onto more level ground. Alluvium. Stream deposits formed in recent geologic time, composed of sand, gravel, and stones. Ammonite. One of a large group of extinct Angular unconformity. A surface separating tilted or folded layers of rock from overlying less disturbed layers. Anticline. An upward Aquifer. A rock layer that is water-bearing. Artesian water. Groundwater that is under sufficient pressure to rise above the level at which it is encountered in a well. It does not necessarily rise completely to the surface. Basalt. An extrusive igneous rock, fine-grained and dark colored, composed mainly of calcium-rich Basement. A name commonly applied to metamorphic or Batholith. A large body of intrusive igneous rock, 40 square miles or more in outcrop area, which extends downward to an unknown depth. Bedrock. The solid rock which underlies soil, sand, clay, or other loose surface material. Belemnite. The cigar-shaped internal shell of an extinct marine Brachiopod. One of a large group of marine shelled animals having two unequal, bilaterally symmetrical shells. Bryozoa. A large group of tiny colonial marine animals that secrete calcareous or horny coverings in a great variety of shapes. Caldera. A large basin-shaped depression caused by explosion or collapse around a volcanic center. Cassiterite. A heavy, brown to brownish black mineral composed of tin and oxygen (SnO2) that is an ore of tin. Cephalopod. A marine Chalcopyrite. A reddish-gold colored ore of copper (CuFeS2). Cirque. A deep, steep-walled recess in a mountain, caused by glacial erosion at the head of a valley. Concretion. A nodular or irregular concentration of minerals such as calcite or limonite, formed by precipitation of the mineral from groundwater around a nucleus. Conglomerate. A rock containing coarse fragments of an older rock, usually as rounded water-worn stones or pebbles. Conodont. One of a group of tiny dark brown tooth-like Diatreme. A volcanic vent or pipe drilled through rocks by the explosive energy of gas-charged molten rock, now containing igneous rock and often altered or unaltered fragments of the surrounding rock. Dike. A vertical or nearly vertical sheet of igneous rock which cuts across the structure of adjacent rocks. Diorite. An intrusive igneous rock composed of sodium-rich Dip. The angle at which a layer of rock is inclined below the horizontal. Dome. A roughly circular upfold in which the rock layers Dowsing. Searching for underground water or ore with a divining rod, usually a forked stick supposed to locate spots where the desired substance may be found under the surface. Echinoderm. One of a large group of marine invertebrate animals, most of which have pentagonal symmetry and a skeleton of many calcite plates. Many forms are spiny. The group includes starfish and sea urchins. Evaporite. Chemical sediments precipitated when water (usually sea water) evaporates. Extrusive rocks. Fault. A break in the rocks in which there has been displacement of the two sides relative to each other. Fault block range. A mountain range bounded on two or more sides by Feldspar. A group of light-colored aluminum silicate minerals that are major constituents of Fold. A bend in rock layers. Foraminiferida. One-celled marine animals with microscopic, perforated, many-chambered calcium carbonate shells, often called forams. Fossil. The remains or traces of an animal or plant which has been preserved in the rock. Fusulinid. One-celled marine animals (forams) with shells which look like a grain of wheat in shape and size, frequently abundant in Colorado Pennsylvanian rocks. Galena. A heavy gray metallic mineral (PbS), often cubic in form, that is the most important ore of lead. Gangue. Nonvaluable minerals occurring in Glaciation. Alteration of the earth’s surface by erosion and deposition by Glacier. A body of ice originating on land by recrystallization of snow, and showing evidence of movement by flowing. Gneiss. A coarse-grained Granite. An intrusive igneous rock consisting essentially of sodium or potassium Graptolite. Extinct marine organisms without known close living relatives, with small black sawblade-like chitinous hard parts preserved as Hematite. A steel gray or metallic grayish black or reddish gray mineral (Fe2O3) that is an important ore of iron. Hogback. A sharp-crested ridge formed by a resistant layer of steeply dipping rock. Huebnerite. A heavy reddish brown mineral (MnWO4) that is a major ore of tungsten. Igneous rocks. Rocks formed by solidification from a molten state, either at the surface (extrusive) or below the surface (intrusive). Intrusive rocks. Joint. A fracture in the rock, along which no discernible movement has taken place. Kerogen. Solid bituminous material in oil shales. Laccolith. A lens-shaped mass of igneous rock intruded into layered rocks. Lava. Fluid or molten rock such as that which issues from a volcano. Lode. A rock mass, often a Massif. A mountainous mass that has relatively uniform geologic characteristics and which may embrace a number of peaks. Mesa. A flat-topped mountain bounded on at least one side by a steep cliff. Metamorphic rock. Rock formed by alteration of pre-existing rock, especially by great temperatures and pressures. Mollusk. Any one of the large group of invertebrate animals which includes the snails, clams, octopuses, squids, and their extinct relatives. Molybdenite. A soft bluish gray, metallic mineral (MoS2) that is a major ore of molybdenum. Monocline. A steplike Moraine. An accumulation of unsorted rock material built up by the action of Native gold. Gold occurring in nature uncombined with other elements. Peneplain. A land surface worn down by erosion to a nearly flat or broadly undulating plain. Petzite. A heavy black or steel gray metallic telluride ore of gold and silver (Ag3AuTe2). Placer. A sand or gravel deposit containing particles or nuggets of gold or other heavy valuable minerals. Plateau. An elevated, comparatively flat surface of land, usually larger than a Porphyry. An igneous rock, usually intrusive, which contains conspicuous large crystals in a fine-grained matrix. Pyrite. A brass-yellow metallic mineral (FeS2) that is an important source of sulfur. It is commonly known as fool’s gold. Reef. A moundlike limestone structure built in the sea by sedentary organisms such as corals. Rhyolite. A light-colored volcanic rock with quartz and Schist. A Sedimentary rocks. Rocks formed of fragments of other rock transported by wind or water, or formed by precipitation from solution. Sphalerite. An amber-yellow to black mineral (ZnS) that is an important ore of zinc. Stalactite. A cylindrical or conical deposit of calcite hanging from the roof of a cavern, formed by evaporation of water droplets containing calcium carbonate. Stalagmite. Columns or ridges of calcite rising from the floor of a cavern, formed by water containing calcium carbonate dripping from a Stock. A mass of igneous intrusive rock that covers less than 40 square miles, has steep sides, and extends to an unknown depth. Tennantite. A metallic gray mineral that contains copper, iron, and arsenic, and is an ore of copper. Tetrahedrite. A brittle, dark gray to black, metallic mineral containing copper, iron, zinc, and silver. Trilobite. One of a primitive group of extinct marine crustaceans, related to crabs and lobsters, having segmented bodies divided by longitudinal grooves into three lobes. Unconformity. A surface separating layers of rock, formed by a period of nondeposition or erosion. Vein. A crack or fissure filled with mineral material, often with valuable ore minerals. |
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