Raymond Wiggers
Geology & Soil-Science Glossary
- Last Updated 13 January 2010 -
Please note that while I always strive for accuracy and applicability, I am not to be held liable in any way for the content, the correctness, or the comprehensiveness of the information provided below. Please let me know at raywiggers@nheg.org. if you spot any mistakes, ambiguities, or glaring omissions.
ABBREVIATION KEY
by = billion years
bya = billion years ago
in = inches
km = kilometers
ky = thousand years
kya = thousand years ago
mi = miles
mm = millimeters
mph = miles per hour
my = million years
mya = million years ago
(Items underlined in the text have their own entries. Terms printed in RED will be defined soon.)
A HORIZON. A soil horizon at or near the top of the soil; i. e., the topsoil. A Horizons contain the bulk of a soil's humus.
ABSAROKA SEQUENCE: The sequence dating from the early Pennsylvanian Period to the early Jurassic Period.
ABYSSAL PLAIN: The broad, flat expanse of the deep-ocean floor, approximately 2-4 mi below sea level.
ACCRETIONARY WEDGE: The thick wedge of sediments that accumulates above a subducting plate.
ACTIVE MARGIN: "The leading edge" of a continent that faces in the direction the continent is moving. An active margin is often the site of faulting, volcanic activity, mountain building, and subduction or obduction.
AEOLIAN (EOLIAN): Referring to the wind or to landforms made by the action of the wind.
AGATE: A form of chalcedony that, due to the presence of mineral impurities, exhibits banding or other patterns considered attractive by collectors.
AGGRADATIONAL COAST: See DEPOSITIONAL COAST.
AIR-FALL DEPOSIT: Tephra deposited by particles falling like rain out of a volcanic cloud, rather than by a pyroclastic flow or pyroclastic surge. 
ALFISOL: A forest soil order, which is partially defined by its relatively thin, light-brown topsoil.
ALKALI FELDSPAR: Any of a group of feldspar minerals that contain sodium and potassium. Cf. PLAGIOCLASE FELDSPAR.
ALLOCHTHONOUS: Referring to a terrane that has been moved from its place of origin to a new location. Cf. AUTOCHTHONOUS and SUSPECT TERRANE.
ALLUVIUM: Sediment laid down in stream valleys.
ALPINE GLACIER: A valley-bound glacier that forms on a relatively steep slope in a mountain environment. Cf. CONTINENTAL GLACIER.
AMMONOID: A type of cephalopod especially prevalent in the Mesozoic Era.
ANDESITE: A fine-grained extrusive rock of chemical composition intermediate between rhyolite and basalt.
ANDISOL: The soil order characterized by a mature soil developed on parent material derived from tephra.
ANHYDRITE: The sulfate mineral CaSO4. It is often associated with evaporite deposits.
ANTECEDENT STREAM: A stream that maintains its course as the surrounding landscape is uplifted, faulted, or folded. An example is a stream that flows directly across a mountain range or plateau because it was there before the modern high surface was pushed upward.
ANTICLINE: A structure in which the crust has been arched upward.
AQUICLUDE: A nonporous or unfractured unit of bedrock or unconsolidated sediments that prevents groundwater from migrating through it.
AQUIFER: A unit of porous or fractured bedrock or unconsolidated sediments that holds groundwater and permits it to migrate through quite freely.
AQUITARD: A unit of bedrock or unconsolidated sediments that permits groundwater to migrate through it, but only very slowly.
ARCHEA (or ARCHAEA): Prokaryotes of the Kingdom Archea; bacterialike organisms of apparently no direct relation to the true bacteria (of Kingdom Eubacteria). The Archea tend to be "extremophiles": they inhabit hypersaline or extremely hot environments such as geothermal vents and brine lakes.
ARCHEAN (or ARCHAEAN) EON: The span of geologic time from approximately 3.8-2.5 bya. N.b. that I define a separate eon preceding the Archean; see HADEAN.
ARIDISOL: A desert or arid-regime soil, which is partially defined by its stony, humus-poor topsoil and its caliche deposits.
ARKOSE: An immature sandstone containing at least 25% feldspar grains.
ASH: Tephra that has a particle diameter of 2 mm or less.
ASH FLOW: See PYROCLASTIC FLOW.
ASH-FLOW TUFF: Tuff formed from ash laid down by a pyroclastic flow.
ASHLAR: Quarried stone that has been cut with smooth faces. Cf. RUBBLE.
ASTHENOSPHERE: The plastic, semisolid zone of the mantle directly below the lithosphere.
ASTROBLEME: Literally, a "star wound." The crater and associated fracturing of the crust caused by the impact of an extraterrestrial object.
ATHENS MARBLE: A trade name for the Niagaran dolostone (NOT true marble) quarried in the Joliet-Lemont area southwest of Chicago.
ATMOSPHERE: The gaseous envelope that exists between a planet's solid or liquid surface and the relative vacuum of outer space.
AULACOGEN: The "failed rift" of a three-rift pattern that forms at a triple junction. While generally the other two rifts continue to widen to create new oceanic crust, the aulacogen remains relatively narrow and fills in with a thick sequence of sediments.
AUTOCHTHONOUS: Referring to a terrane that has remained in its original setting. Cf. ALLOCHTHONOUS and SUSPECT TERRANE.
B HORIZON: The relatively humus-poor horizon that lies below a soil's A horizon or E horizon. It is a layer that accumulates colloids that have migrated downward from overlying horizons.
BACKREEF DEPOSITS: Sediments or rock situated at the rear of a reef.
BAJADA: A very gentle slope of unconsolidated sediments formed by the merging of alluvial fans at the base of a mountain front.
BALLAST: Small pieces of stone, of a fairly uniform size, used as the stable foundation for railroad tracks.
BALLISTIC BLOCK: A large rock fragment explosively ejected in a ballistic trajectory from a volcano.
BASALT: A fine-grained, quartz-poor, and usually dark-colored igneous rock. It is the extrusive equivalent of gabbro.
BASIN-AND-RANGE PROVINCE: A very large area of western North America (including Trans-Pecos Texas) where the crust is being stretched apart, perhaps by the stresses generated by the Pacific and North American plates sliding by one another along California's San Andreas Fault. Basin-and-Range terrain is characterized by fault-block mountains (horsts) and normal faults bordering down-dropped grabens (bolsons).
BEDFORD LIMESTONE: See INDIANA LIMESTONE.
BEDROCK: Any section of rock that is still attached to the crust. Cf. OVERBURDEN.
BIOCALCARENITE: A type of limestone chiefly composed of very small shell fragments and tiny whole fossils. Cf. COQUINA.
BIOHERM: A mound or other raised structure largely built by lime-secreting marine organisms. Some ecologists and other experts prefer this term to the vaguer and much-disputed term reef.
BLOB TECTONICS: A theory that seeks to account for the massive upwelling of magma and the construction of very large volcanic features on such terrestrial planets as Mars or Venus without the existence of well-defined, moving plates and without the formation of distinct ocean basins and continents.
BOLIDE: Strictly speaking, a large meteor that explodes in the atmosphere. In other words, it doesn't strike the ground or create a crater. However, the term has been appropriated by some to also mean a large meteorite that does indeed hit the surface to produce a sizeable crater.
BOLSON: A closed basin, typical of the Basin and Range Province, that is ringed by fault-block mountains.
BOMB: An extrusive rock, often aerodynamically streamlined, that is explosively ejected from a volcano while still molten; it solidifies in midair.
BOULDER: A detached rock larger than 256 mm in diameter.
BOUNDSTONE: A type of carbonate rock in the Dunham Classification System that is composed of sediments bound together by a framework of organisms (corals, bryozoans, sponges, stromatolites, etc.)
BRACHIOPOD: A type of marine animal with two valves (shells) that superficially resemble those of bivalves (clams, oysters, etc.). However, brachiopods belong to a completely different lineage of animals (Phylum Brachiopoda).
BRECCIA: A clastic rock composed of coarse, angular rock fragments. Cf. CONGLOMERATE.
BRICK: A shaped, rectangular unit of building material made of fired clay (which is sometimes combined other substances).
BROWNSTONE: An architectural term for a red-, brown-, or maroon-tinted sandstone. This type of dimension stone was particularly popular with architects who adhered to the late-nineteenth-century Richardsonian Romanesque style.
BRYOZOAN: A tiny colonial marine animal belonging to Phylum Bryozoa, the "moss animals."
BUTTE: A relatively narrow, flat-topped hill composed of essentially horizontal strata. Cf. MESA.
C HORIZON: A soil horizon, located below the A horizon, that is only slightly altered parent material.
CAISSON: In architecture, a timber-lined vertical shaft filled with concrete or other hard material. This shaft serves as an anchor which connects a building to the underlying hardpan. In Chicago, this construction technique generally predates the time when larger building were directly connected with the bedrock under the unconsolidated sediments.
CALCAREOUS: Containing calcium carbonate.
CALCITE: The mineral equivalent of calcium carbonate (CaCO3) or lime.
CALDERA: A broad, bowl-shaped landform created by the collapse of a volcano due to the emptying of its underground magma chamber.
CALICHE: A deposit or layer of calcium carbonate found in Aridisols.
CAMBRIAN PERIOD: The span of geologic time from approximately 544-505 mya.
CATENA: The expression of variations found within one basic soil type; these variations are caused by differing relief, steepness of slope, amount of moisture, and other facrtors.
CARBONATE MINERAL: A member of a chemical group of minerals that contain, among other substances, - CO3.
CARBONATE PLATFORM: A shallow, marine shelf of carbonate sediments. Carbonate platforms exist only in tropical or subtropical waters; one modern example is the Bermuda Banks.
CARBONATE ROCK: A rock primarily composed of such carbonate minerals as calcite (lime, calcium carbonate) or dolomite (magnesium carbonate).
CARBONIFEROUS PERIOD: The European designation for the Mississippian and Pennsylvanian Periods grouped together. This was the span of geologic time from approximately 360-286 mya.
CARRARA MARBLE: A world famous, lustrous, white marble quarried in Carrara, in northern Italy. This rock type was originally limestone of the early Jurassic Period; it was subsequently metamorphosed into marble in the Tertiary Period.
CENOZOIC ERA: The span of geologic time from approximately 65 mya to the present.
CEPHALOPOD: A marine mollusc of the Class Cephalopoda. Members of this class include the nautiloids, octopi, squid, and the now-extinct ammonoids.
CHALCEDONY: A microcrystalline, translucent, gray to brown form of quartz that often has a waxy luster. It is usually deposited in rock cavities by aqueous solutions.
CHALK: A carbonate rock composed primarily of the tests ("shells") of organisms called coccolithophorids. In the Dunham Classification System, chalk is an example of a lime mudstone.
CHAMPLAIN BLACK: A striking black limestone, often called "black marble," quarried on Lake Champlain's Isle la Motte from the Crown Point Limestone of the Chazy Group. This stone is Ordovician in age.
CHARNOCKITE: A granitelike rock of igneous intrusive origin that consists predominantly of the minerals microcline and quartz.
CHEMICALLY PRECIPITATED ROCK: Referring to a sedimentary rock type that is formed by the precipitation of mineral crystals from seawater. Cf. CLASTIC ROCK.
CHERT: A very hard rock composed of microcrystalline quartz.
CHLORITE: An often greenish mineral associated with low-grade metamorphic rocks. Its chemical formula is (Mg6-x-y Fey Alx) O10 (OH)8.
CLADDING: Stone used on the exterior of building for decorative effect. Usually it is not a load-bearing element.
CLASTIC ROCK: Referring to a sedimentary rock type that is formed of cemented particles -- boulders, cobbles, pebbles, sand, silt, or clay -- originally produced by the weathering or erosion of other, older rock. Cf. CHEMICALLY PRECIPITATED ROCK.
CLAY: A mineral particle less than .003 mm (.0001 in) in diameter.
COBBLE: A rock particle 64-256 mm (2.5-10 in) in diameter.
COLLOID: Any clay-sized particle.
COLUMN: A speleothem that is a stalagmite and a stalactite that have fused into one vertical shaft that extends from a cave's ceiling to its floor. It is one form of dripstone.
COMPOSITE VOLCANO: See STRATOVOLCANO.
COMPRESSIONAL FORCE: A force that pushes togther a portion of the crust. Cf. TENSIONAL FORCE.
CONGLOMERATE: A clastic rock composed of coarse, rounded rock fragments. Cf. BRECCIA.
CONNECTICUT VALLEY BROWNSTONE: Red-, brown-, or maroon-tinted stone quarried from the Portland Arkose Formation of the Connecticut River Valley of Connecticut and Massachusetts. This is the architectural brownstone commonly encountered in New York City and other eastern cities. The Portland Arkose is early Jurassic in age. Cf. LAKE SUPERIOR BROWNSTONE.
CONNEMARA MARBLE: Marble from Connemara Island, Ireland. The most frequently encountered form is green; it dates from the later Proterozoic Eon, roughly 680 million years ago. The black form (also called Galway Marble) dates to the late Permian Period. The spelling "Connemargh" is incorrect and was probably copied from an out-of-date source.
CONTINENTAL CRUST: The part of the Earth's crust that forms the continents. This crust is largely composed of felsic rocks.
CONTINENTAL DRIFT: A theory, now superseded by the more modern theory of plate tectonics, that sought to explain the motion of continents over the Earth's surface.
CONTINENTAL GLACIER: A large, broad glacier that forms on a relatively shallow slope in lowland environments. Cf. ALPINE GLACIER.
CONTINENTAL SHELF: An extension of a continental margin covered by relatively shallow salt water.
CONTINENTAL SLOPE: The slope that connects the shallow continental shelf to the deep abyssal plain.
CONVERGENT PLATE BOUNDARY: A zone where two plates move toward each other.
COQUINA: A type of limestone that mainly composed of shell debris from marine organisms. The individual bits of shell debris are often much larger than sand grains. They are cemented together with calcite.
CORE: The center zone of the Earth's interior, about 3500 km (2200 mi) in diameter, just below the mantle. It is composed of a liquid iron-nickel outer core and a solid iron-nickel inner core.
COURSE: A horizontal row of bricks or dimension stone.
CRATON: The stable interior of a continent.
CRETACEOUS PERIOD: The span of geologic time from approximately 146-65 mya.
CROSSBEDDING: Patterns of curving lines or traces found within the strata of sandstone and other sedimentary rocks. Crossbedding indicates the general direction and force of the wind or water that originally laid down the sediments.
CRUST: The uppermost section of the solid Earth, ranging in thickness from about 6-50 km (4-30 mi). It lies directly above the mantle and directly below the atmosphere.
CRYSTALLINE CARBONATE: A type of carbonate rock in the Dunham Classification System that is composed of interlocking carbonate crystals that formed after its sediments were first laid down.
CUESTA: A ridge with an asymmetrical profile (one side gently sloping; the other steep) formed by the exposure of dipping strata. Cf. HOGBACK.
CYANOBACTERIUM (plural = CYANOBACTERIA): A relatively large and advanced photosynthetic bacterium. A member of the Kingdom Eubacteria, it is sometimes inaccurately called a "blue-green alga."
DEBRIS FLOW: A deposit formed by a slurry of water, mud, and larger rock fragments.
DEGRADATIONAL COAST: See EROSIONAL COAST.
DEPOSITIONAL COAST: A shoreline where, on average, more sediments (sand or gravel) are deposited than removed.
DESERT PAVEMENT: A pavementlike mantle of loose stones on a windswept desert surface. These stones are derived from a preexisting alluvium deposit. Cf. HAMMADA and REG.
DESERT VARNISH: The black or reddish coating of clay and manganese and iron oxides that often covers rocks in a desert environment.
DEVONIAN PERIOD: The span of geologic time from approximately 410-360 mya.
DIKE: A narrow, often vertical and wall-like mass of intrusive rock that forms when upward-moving magma cuts across preexisting rock bodies.
DIMENSION STONE: Any quarried rock product that is cut to a specific use for architectural or construction uses.
DIP: A stratum's angle of tilt, measured from the horizontal. Cf. STRIKE.
DISAPPEARING STREAM: A stream in a karst landscape that has had part of its course diverted into a subterranean channel.
DIVERGENT PLATE BOUNDARY: A zone, characterized by rifts and volcanic activity, where the older portions of two plates are moving away from one another.
DOLOMITE: The carbonate mineral CaMg(CO3)2. Also, used to denote the rock type chiefly made up of this mineral. Cf. DOLOSTONE.
DOLOSTONE: The more modern and less ambiguous term for the rock type chiefly composed of the mineral dolomite.
DOME: A structure in the crust, more or less round as seen from above, in which rock units have been arched upward.
DRIFT: A collective term for material deposited directly or indirectly by a glacier. While till and outwash are included in this definition, loess is not.
DRIPSTONE: Any type of speleothem formed by the precipitation of calcite and other minerals from dripping water.
DRUMLIN: An asymmetrical hill, composed of either drift or bedrock mantled with drift, that is created by an advancing glacier. Drumlins, which often occur in large swarms, often have a shallow slope pointing in the direction of glacial movement, and a steep backslope.
DUNHAM CLASSIFICATION SYSTEM: A system of terminology that describes different types of carbonate rocks according to their texture and deposition. See BOUNDSTONE, CRYSTALLINE CARBONATE, GRAINSTONE, LIME MUDSTONE, PACKSTONE, and WACKESTONE.
E HORIZON: A soil horizon, often white or pale yellow in color, that almost exclusively contains quartz sand.
END MORAINE: A ridge of till deposited at the leading margin of a glacier when the rate of ice melting at the margin is matched by new ice moving up from the rear.
ENTISOL: A very young soil order with only an A horizon,
EOCENE EPOCH: The span of geologic time from approximately 54-38 mya.
EOLIAN: See AEOLIAN.
EPIPEDON: A soil's A Horizon (surface layer or topsoil).
EROSION: The process by which sediments and other earth materials are removed by wind or running water.
EROSIONAL COAST: A shoreline where, on average, more sediments (sand or gravel) are being removed than deposited.
ERRATIC: A detached rock that has been transported by a glacier.
ESKER: A long, snaking, steep-sided ridge formed by the deposition of sand, gravel, or cobbles in a crevasse or stream tube of a stagnant glacier.
EUBACTERIA: Prokaryotes of the Kingdom Eubacteria. The "true bacteria"; cf. ARCHEA.
EUKARYOTE: An organism that has highly organized cell structure and such organelles as a nucleus and mitochondria.
EVAPORITE: A rock or mineral that has formed from the evaporation of salt brine in a lagoon, closed basin, or similar environment.
EXTRUSIVE ROCK: Igneous rock formed by the cooling of lava or tephra on the Earth's surface (either on dry land or on the floor of a sea or lake).
FACIES: Literally, "appearance" or "aspect"; the overall set of characteristics displayed by a rock type that indicates the environment in which it formed.
FACING STONE: See CLADDING.
FAULT: A fracture in bedrock where there has been significant displacement between the two sides. Cf. JOINT.
FAULT-BLOCK MOUNTAINS: A mountain range formed when tensional forces in the Earth's crust create a horst bounded by normal faults and grabens. The mountains are situated on the horst.
FELDSPAR: A term for any member of the very prevalent complex of silicate minerals that include aluminum, silicon, and oxygen.
FELSIC: Referring to rocks rich in silicon and aluminum. Such rocks include granite and rhyolite.
FIELD STONE: Detached rocks (in our area, usually erratics) that are collected for architectural and construction purposes from farm fields and other open ground.
FLATIRON: A steeply dipping, triangular landform situated between two small valleys that cut through a hogback.
FLOATING FOUNDATION: The raftlike structure, set in unconsolidated sediments, on which a building is constructed. In Chicago, this construction technique generally predates the time when larger building were directly connected to the bedrock.
FLOW: A form of mass wasting in which Earth materials lubricated by a substantial amount of water move downslope.
FLOWSTONE: A speleothem formed by the precipitation of calcite and other minerals from flowing water. Flowstones take on many different sheetlike or rounded shapes.
FLYSCH: A sequence of deep-water sedimentary rocks (especially shales and turbidites) that accumulate in a foreland basin. Cf. MOLASSE.
FORAMINIFER or FORAM: A type of planktonic marine organism, usually microscopic or almost so, that grows a calcareous test ("shell").
FOREARC BASIN: A basin situated between an island arc and an accretionary wedge.
FORELAND BASIN: A basin, often filled with ocean water, that forms between an island arc and a continent's craton.
FOREREEF DEPOSIT: The sloping fan of talus that is just seaward of the main body of a reef. In fossil reefs, the forereef is usually composed of sloping strata of limestone breccia and other surf and storm debris.
FOSSIL: Originally, a term for any object dug out of the ground. In modern parlance, it refers specifically to a preserved part or indication of an ancient organism's form or behavior.
GABBRO: A coarse-grained, dark-colored, and quartz-poor igneous rock. It is the intrusive equivalent of basalt.
GASTROPOD: A mollusc of the Class Gastropoda. Members of this class include the snails.
GELISOL: A soil order characterized by the presence of permafrost.
GEODE: A spherical or ovoid rock with a hollow center lined with crystals. Some geodes even contain petroleum.
GEOTHERMAL VENT: A vent located along a mid-ocean ridge from which rise hot, mineral-rich waters.
GLACIAL DRIFT: See DRIFT.
GLACIATION: An episode (often lasting about 100 ky) consisting of the formation, advance, and retreat of one continental ice sheet. Not synonymous with ICE AGE (which see).
GLACIER: A large, persistent mass of ice that moves outward under the weight of its own thick center on low slopes, or downward on steep valley slopes. A glacier forms when there is, for an extended period, more net annual snowfall than net annual melting.
GNEISS: A high-grade metamorphic rock characterized by linear bands of dark and light minerals.
GONDWANALAND: The Paleozoic-Era supercontinent that included Africa, India, South America, Australia, and Antarctica.
GRABEN: A down-dropped block bordered by normal faults and horsts.
GRADED BED: A bed of sediments that changes, from bottom to top, from coarse particles to fine particles.
GRAINSTONE: A type of carbonate rock in the Dunham Classification System that is composed of grains that are not surrounded by a mud matrix.
GRANITE: A coarse-grained and light-colored igneous rock. The intrusive equivalent of rhyolite, it contains at least 10% quartz.
GRANITELIKE ROCK: A rock that resembles true granite but has a somewhat different mineral content. Examples include syenite and granodiorite.
GRANODIORITE: A rock resembling granite that has most of its feldspar content in the form of plagioclase.
GRAVEL: A general term for rock particles of pebble size.
GRAYWACKE (GREYWACKE): An immature sandstone that contains 15% or more clay particles.
GROUND MORAINE: A thin, flat mantle of till deposited by a retreating glacial margin.
GROUNDWATER: Water held beneath the Earth's surface.
GYPSUM: The sulfate mineral CaSO4-2H2O. It is often associated with evaporite deposits.
HADEAN EON: The span of geologic time from approximately 4.6-3.8 bya.
HALIDE MINERAL: A member of a chemical group of minerals that contain, among other substances, a halide element (Fluorine, Chlorine, Bromine, etc.).
HALITE: The halide mineral NaCl, also known as "rock salt."
HAMMADA (also spelled HAMADA): A stony, windswept desert surface, where the stones are boulder-sized or nearly so, and thus especially difficult for travelers to negotiate. Some geomorphologists distinguish a hammada from a reg by stating the former is windswept bedrock rather than loose rock. As though this weren't ambiguity enough, the term hammada may also be restricted to the stony surfaces of such flat, upland surfaces as desert mesas and plateaux. Also cf. DESERT PAVEMENT.
HARDPAN: A zone of soil or unconsolidated sediments that has been compacted into a stiffer, rocklike consistency.
HIGH-GRADE METAMORPHIC ROCK: A metamorphic rock that has been subjected to a relatively high increase in pressure and to temperatures above 320 degrees Centigrade.
HISTOSOL: A wetland soil order composed almost exclusively of peat or other organic matter. Its only horizon is an O horizon.
HOGBACK: A knife-edge ridge formed by the exposure of steeply dipping strata. Cf. CUESTA.
HOLOCENE EPOCH: The span of geologic time from approximately 10 kya to the present.
HORIZON: One distinct layer of an entire soil profile.
HORST: An upthrust block bordered by normal faults and grabens.
HUMUS: Organic matter that is almost completely decomposed. Cf. PEAT.
HYPERSALINE: Extremely salty.
ICE AGE: A major, extended event in Earth history (often lasting 10 my or more) that includes at least several glaciations and interglacials.
ICHNOFOSSIL: See TRACE FOSSIL.
IGNEOUS ROCK: A rock formed by the cooling of magma, either underground or at the Earth's surface.
IGNIMBRITE: A rock formed from the pumice and other materials deposited by a pyroclastic flow.
ILLITE: One type of clay that is a secondary mineral derived from the weathering of primary minerals. Illite in turn weathers quite readily into other compounds.
IMMATURE SANDSTONE: A sandstone that a significant percentage of angular particles, minerals other than quartz, and particle sizes other than sand. These constituents indicate that the material that makes up the sandstone comes from a recently eroded, geographically close source.
IMPACT SAG: A depression formed in a surface where a ballistic block lands.
INCEPTISOL: A young soil order that has only an A horizon and a weakly developed B horizon.
INDIANA LIMESTONE: The widely used dimension stone quaried in the vicinity of the towns of Bedford and Oolitic, in southern Indiana. This stone is biocalcarenite and it belongs to the Mississippian-Period Salem Limestone Formation. It is approximately 340 million years old.
INTERGLACIAL: The relatively short (often10-15 ky) warm period between two glaciations.
INTRUSIVE ROCK: Igneous rock formed from magma cooling beneath the Earth's surface.
ISLAND ARC: A curving line of volcanic islands that forms over a subducting plate.
JOINT: A fracture in bedrock where there has been no significant displacement between the two sides. Cf. FAULT.
JOLIET MARBLE: A trade name for the Niagaran dolostone (NOT true marble) quarried in the Joliet-Lemont area southwest of Chicago.
JURASSIC PERIOD: The span of geologic time from approximately 208-146 mya.
KAME: A conical, ridgelike, or terracelike landform made of sand, gravel, or cobbles deposited in a depression or cavity in a stagnant glacier, or at the base of a waterfall on the margin of a stagnant glacier.
KAOLINITE: One type of clay that is a secondary mineral derived from the weathering of primary minerals. Kaolinite is often quite resistant to further weathering, and has properties that make it an excellent modeller's and potter's clay.
KARST: A terrain of carbonate rocks characterized by such solution features as sinkholes, caves, and disappearing streams.
KASKASKIA SEQUENCE: The sequence dating from the early Devonian Period to the early Pennsylvanian Period.
KETTLE: A type of glacial landform. When a glacial margin retreats and leaves a detached block of ice behind, the block is subsequently buried or partially so by outwash. When the ice block finally melts, it leaves an indentation or depression in the outwash. Kettles are then often filled with water and become lakes or ponds.
KNICK POINT: An abrupt change in the slope of a stream bed. The most obvious example of a knick point is a waterfall.
LACCOLITH: A domed or mushroom-shaped body of intrusive rock formed when upward-moving magma lifts up preexisting strata.
LAKE SUPERIOR BROWNSTONE: Red-, brown-, or maroon-tinted sandstone quarried from the Jacobsville Sandstone Formation of Michigan's Upper Peninsula, or from the Bayfield Group of northern Wisconsin. These sandstones, originally though to be late Cambrian in age, are actually Proterozoic approximately 1.0 billion years old. This is the architectural brownstone commonly encountered in Chicago and other Midwestern cities. Cf. CONNECTICUT VALLEY BROWNSTONE.
LAHAR: A volcanic and often highly destructive mudflow formed by the mixing of water and ash.
LANDFORM: Any distinct feature on a planet's surface produced by natural forces.
LAPILLI: Tephra particles that have a diameter of 2-64 mm (0.1-2.5 in). In other words, they're larger than ash particles.
LAPILLI TUFF: An igneous, extrusive rock composed of lapilli that are welded or cemented together.
LARAMIDE OROGENY: The mountain building event, approximately 80-50 mya, that created the Ancestral Rockies in the American West.
LATERITE: A clayey material, often reddish or orange in color, that is derived from heavily weathered rock and is characteristic of wet, tropical climates. It contains high concentrations of aluminum and iron compounds.
LAURASIA: The late-Paleozoic-Era supercontinent that included North America and Eurasia.
LAURENTIA: The smaller, ancestral version of North America as it existed in the Paleozoic Era.
LAVA: Molten rock at the Earth's surface (either on land or on the ocean bottom). Cf. MAGMA.
LAVA DOME: A dome or domelike hill composed of solidified viscous lava (usually rhyolite).
LAVA FLOW: A body of still-molten lava moving down a slope, or the landform made from such a body of lava after it has solidified.
LIME MUDSTONE: A type of carbonate rock in the Dunham classification system that is mostly composed of clay-sized, lime-mud particles; grains make up less than 10% of the rock. One example of a lime mudstone is chalk.
LIMESTONE: A sedimentary rock chiefly composed of the mineral calcite. This rock is often chemically precipitated, but it may clastic instead. Cf. DUNHAM CLASSIFICATION SYSTEM.
LITHIFED: Referring to sediments that have been turned into rock.
LITHOSPHERE: The brittle, solid uppermost section of the solid Earth, comprising the crust and uppermost mantle.
LOAD-BEARING ELEMENT: A portion of a building that must directly bear the weight of the part of the structure above it.
LOESS: A type of silt of calcareous composition that was originally deposited in stream valleys by glacial meltwater and subsequently blown by the wind to upland locations.
LOW-GRADE METAMORPHIC ROCK: A metamorphic rock that has been subjected to a relatively gentle increase in pressure and to temperatures between 200 and 320 degrees Centigrade.
MAFIC: Referring to dark-colored igneous rocks that are rich in iron and magnesium. Such rocks include basalt and gabbro.
MAGMA: Molten rock below the Earth's surface. Cf. LAVA.
MAGMA CHAMBER: A subterranean cavity that serves as the reservoir for magma below a volcano or volcanic vent.
MANTLE: The zone of the Earth's interior, about 1800 mi deep, that lies under the crust.
MARBLE: The metamorphic equivalent of such carbonate rocks as limestone and dolostone. It has been subjected to high temperatures, and is widely used in sculpture and cladding because it takes a high polish.
MARL: A limestone with a high clay content.
MASS WASTING: The process by which Earth materials move down a slope due to the force of gravity.
MATURE SANDSTONE: A sandstone that contains almost exclusively well-rounded, sand-sized quartz particles. The lack of angular grains, other particle sizes, and minerals other than highly resistant quartz suggests the material that makes up the sandstone comes a anciently eroded, geographically distant source.
MELTWATER: Liquid water produced from the melting of a glacier.
MESA: A broad, flat-topped hill resembling a small plateau, which is made up of essentially horizontal strata. Cf. BUTTE.
MESOZOIC ERA: The span of geologic time from approximately 245-65 mya.
METAMORPHIC ROCK: A rock that has been subjected to increased temperature, increased pressure, or both, and therefore has been transformed into a rock type different than its original form.
METEOR: An object of interplanetary origin that burns up in the atmosphere without reaching the surface.
METEOROID: An object of small dimensions and interplanetary origins moving through space.
METEORITE: An object of interplanetary origin that reaches the surface of the Earth without completely burning up in the atmosphere.
MICROCLINE: An alkalai feldspar mineral of the composition KAlSi3O8.
MICROCRYSTALLINE: Referring to crystals that are too small to be seen with the naked eye.
MID-OCEAN RIDGE: A narrow range of undersea volcanoes, often extending for thousands of miles, that is formed by a spreading center at a divergent plate boundary.
MINERAL: A naturally occurring, inorganic substance. Many minerals form crystals.
MIOCENE EPOCH: The span of geologic time from approximately 23-5 mya.
MISSISSIPPIAN PERIOD: The span of geologic time from approximately 360-325 mya. See CARBONIFEROUS PERIOD.
MOLASSE: A sequence of terrestrial and shallow marine sedimentary rocks (especially sandstones) that accumulate in a foreland basin after it is filled in with flysch deposits.
MOLLISOL: A prairie or grassland soil order, partially defined by its deep, dark-brown to black topsoil.
MONOCLINE: An asymmetrical bend in the crust where strata are horizontal, then dip downward, then become horizontal again at a lower level.
MORAINE: See GROUND MORAINE and TERMINAL MORAINE.
MOUNT AIRY GRANITE: White to grayish granite of excellent quality for cladding. It is quarried in Mount Airy, North Carolina, on the eastern side of the Blue Ridge. It is Silurian to Devonian in age.
MUDFLOW: A form of flow in which mud is the primary type of Earth material moving downslope.
MUDSTONE: See LIME MUDSTONE.
MUSCOVITE: A light-colored, sheet-forming silicate mineral of the mica group. Its chemical formula is KAl2(Si3Al)O10(OH,F)2.
NATIVE-ELEMENT MINERAL: A member of a chemical group of minerals that each contain only a single element (elemental copper, elemental sulfur, etc.).
NECK (VOLCANIC NECK): A resistant plug that has become exposed by the erosion of the volcano that originally surrounded it.
NEOGENE PERIOD: The span of geologic time from approximately 24 mya to the present.
NIAGARAN DOLOSTONE: Dolostone of Silurian age. This the bedrock nearest the surface in most of the Chicago region. It is also found in other parts of the Great Lakes basin, including Niagara Falls.
NORMAL FAULT: An inclined fracture in the crust in which the upper block (or hanging wall) slides down relative to the lower block (or footwall).
NOVACULITE: A dense, white or light-colored form of chert.
O HORIZON: A soil horizon composed almost exclusively of organic matter. Two examples of O horizons are leaf litter and a peat deposit in a wetland.
OCEANIC CRUST: The part of the Earth's crust that floors ocean basins. This crust is largely composed of mafic rocks.
OBDUCTION: The process of plate collision involving two continents. Because continental crust is not dense enough to sink into the Earth's interior, a highly contorted zone of high mountains forms.
OBLIQUE-SLIP FAULT: An inclined fracture in the crust in which the upper block (or hanging wall) slides both sideways and up or down relative to the lower block (or footwall).
OLIGOCENE EPOCH: The span of geologic time from approximately 38-23 mya.
OPHIOLITE: A term used to denote a succession of rock types including deep-ocean sedimentary strata, basalts, and ultramafic rocks.
ORBICULAR GRANITE: A granite that has its crystals arranged in a radial pattern, often with light-colored minerals in the center and the darker minerals arranged concentrically around them toward the exterior. This term is sometimes confused with rapakivi.
ORDER: In biology, the taxonomic level that is a group of closely related families of organisms. In pedology, the highest, most generalized taxonomic level used in the modern U.S.D.A. soil-classification system. In this system there are twelve soil orders, each denoted by a name ending in -sol.
ORDOVICIAN PERIOD: The span of geologic time from approximately 505-440 mya.
OROGENY: A mountain-building event.
ORTHOCLASE: An common alkali-feldspar mineral: K(AlSi3O8).
OUACHITA OROGENY: The mountain-building event, approximately 300 mya, which created the Ancestral Ouachita Range in Texas and other South-Central and Southeastern states.
OUTCROP: An exposure of bedrock.
OUTWASH: A deposit of sorted sand, pebbles, and cobbles carried and then deposited by glacial meltwater streams.
OVERBURDEN: Any loose material that sits atop the bedrock: soil, unconsolidated sediments, etc. Also, any bedrock that lies over a desired vein of coal or ore.
OXIDE MINERAL: A member of a chemical group of minerals that contain, among other substances, -O2.
OXISOL: An ancient, highly weathered soil order characterized by an orange or reddish color and a buildup of iron and aluminum oxides. Oxisols are usually found in humid tropical environments.
PACKSTONE: A type of carbonate rock in the Dunham Classification System that contains more than 50% grains and some lime mud.
PALEOCENE EPOCH: The span of geologic time from approximately 65-54 mya.
PALEOGENE PERIOD: The span of geologic time from approximately 65-24 mya.
PALEOSOL: Literally, an "ancient soil" that is later buried by younger rock units or unconsolidated sediments.
PALEOZOIC ERA: The span of geologic time from approximately 544-245 mya.
PANGEA (PANGAEA): The supercontinent that existed from approximately 300-200 mya. Pangea included both Laurasia and Gondwanaland.
PANNOTIA: The supercontinent that included most modern continents and that existed from approximately 650-550 mya.
PARENT MATERIAL: The unconsolidated material from which a true soil evolves.
PASSIVE MARGIN: The "trailing edge" of a continent. It is usually tectonically quiet, with no faulting, mountain building, volcanic activity, subduction, or obduction. Instead, the sustained deposition of sediments from the continental interior forms a broad continental shelf.
PATCH REEF: A small bioherm that forms in the lagoon behind the main reef.
PEAT: Organic matter that is only partially decomposed. It is usually deposited in a wetland environment. Cf. HUMUS.
PEBBLE: A rock particle 2-64 mm (.08-2.5 in) in diameter.
PEDIMENT: The gentle, concave slope below the steepest part of a mountain front. It is an erosional bedrock feature, but it is usually mantled by rock debris.
PEDOLOGY: The study of soils; soil science.
PELÉAN ERUPTION: A violent volcanic eruption that produces pyroclastic flows.
PENNSYLVANIAN PERIOD: The span of geologic time from approximately 325-286 mya. See also CARBONIFEROUS PERIOD.
PERIDOTITE: A dense, dark-toned, ultramafic rock thought to originate in the Earth's mantle.
PERIGLACIAL: Adjacent to or near a glacier. Cf. PROGLACIAL.
PERMIAN PERIOD: The span of geologic time from approximately 286-245 mya.
PERMAFROST: The subsurface zone of permanently frozen soil found in periglacial and other cold, high-latitude or high-altitude environments.
PERMINERALIZED FOSSIL: A fossil in which the remains of an organism are surrounded by a matrix of minerals that have precipitated out of water or another fluid. Cf. REPLACEMENT FOSSIL.
PHANEROZOIC EON: The span of geologic time from approximately 544 mya to the present.
PHOSPHATE MINERAL: A member of a chemical group of minerals that contain, among other substances, -PO4.
PHREATIC ERUPTION: A violent volcanic eruption involving expanding steam derived from seawater, groundwater, or water originally contained under pressure in the magma.
PHYLLITE: A low-grade, foliated metamorphic rock that has a silky sheen due to the presence of the minerals muscovite or chlorite.
PILLOW LAVA: A lava (usually basalt) that erupts and hardens under water to form a pillow- or egg-shaped extrusive rock.
PIT: An open-air facility where unconsolidated sediments (sand, gravel, etc.) are mined. Cf. QUARRY.
PLAGIOCLASE FELDSPAR: Any of a group of feldspar minerals that contain sodium, calcium, or both. Plagioclase feldspars are sometimes called "soda-lime feldspars."
PLATE: One of about fifteen rigid sections that make up the Earth's lithosphere. A plate can contain oceanic crust only, or both oceanic and continental crust.
PLATE TECTONICS: The theory first developed in the middle of the twentieth century that explains various geological phenomena in the context of moving plates.
PLAYA: The lowest part of a bolson. After a rain, it often holds an ephemeral lake.
PLINIAN ERUPTION: A violent volcanic eruption characterized by a huge volume of tephra shot high into the stratosphere.
PLINTH: The basal part of a building's exterior; the lowermost course of stone on a building's exterior.
PLEISTOCENE EPOCH: The span of geologic time from approximately 2 mya-10 kya.
PLIOCENE EPOCH: The span of geologic time from approximately 5-2 mya.
PLUG: A deposit of lava that solidifies in a vent.
PORPHYRY: An igneous rock containing large mineral crystals embedded in a matrix.
POTHOLE: A circular or oval depression in stone, asscociated with a waterfall. Many potholes are formed by resistant loose rocks that swirl in confined area at the base of cascading water. In other words, the developing pothole is the mortar; the swirling stones are the pestle.
POUR-OFF: A Trans-Pecos Texas term for a waterfall. Pour-offs are usually dry unless there is enough rainfall or snow melt to cause water to pour off them.
PRECAMBRIAN TIME: The span of geologic time from approximately 4.6bya-544 mya. It comprises the Hadean, Archean, and Proterozoic Eons.
PRIMARY MINERAL: A mineral present in parent materials or soils that have not yet been significantly weathered.
PROFILE: The entire vertical sequence of a soil's horizons.
PROGLACIAL: Referring to a lake, stream, or other feature in front of a glacier. Cf. PERIGLACIAL.
PROKARYOTE: An organism that lacks highly organized cell structure, and which does not have such organelles as a nucleus or mitochondria.
PROTEROZOIC EON: The span of geologic time from approximately 2.5 bya-570mya.
PROTIST: A member of the Kingdom Protista.
PROTISTA: The kingdom of eukaryotic organisms that includes the algae, protozoa, and slime molds.
PUMICE: A volcanic glass of frothy texture that contains many vesicles.
PYROCLASTIC FLOW: In a general sense, an extremely hot (ca. 10000 F) cloud of particles and gas that move down a volcano's slope at up to 600 mph. In a more restricted sense, it is a hot cloud of particles and gas that has laminar flow, i. e., it moves downhill as a ground-hugging sheet.
PYROCLASTIC SURGE: A form of pyroclastic flow in which the hot cloud of particles and gas that exhibit turbulent flow -- i. e., a billowing form.
QUARRY: An open-air facility where the local bedrock is mined. Cf. PIT.
QUARTZ: An extremely prevalent mineral composed of silicon and oxygen. Classified as either a silicate or oxide mineral, its chemical formula is SiO2.
QUARTZ DIORITE: A granitelike igneous intrusive rock containing 20-60% quartz.
QUARTZITE: Either a metamorphic equivalent of sandstone (metaquartzite), or a metamorphosed sandstone with grains cemented with silica (orthoquartzite).
QUATERNARY PERIOD: The span of geologic time from approximately 2 mya to the present. This older division of geologic time and the preceding Tertiary Period are gradually being supplanted by the system using the Paleogene Period and the Neogene Period instead.
RADIAL DIKE: A dike that emanates like the spoke of a wheel from the central vent of a volcano.
RAPAKIVI: A term for igneous rock containing rounded and usually pink potassium-feldspar
crystals that are borders with white plagioclase-feldspar or alkali-feldspar zones.
REEF: This term has been the subject of intense semantic squabbling amongst marine ecologists, paleontologists, and geologists. Any definition one uses will be attacked by someone else. A reef is any submerged structure on which a ship may run aground; an organic reef is a marine structure largely made of lime-secreting organisms. See BIOHERM.
REG: A stony, windswept desert surface. The stones are usually loose rock fragments considerably smaller than boulders. Cf. DESERT PAVEMENT and HAMMADA.
REGOLITH: Pulverized rock material that serves as the mineral and nonorganic constituent of true soil.
REPLACEMENT FOSSIL: A fossil in which the original remains of an organism are replaced, often in very fine detail, by minerals that have precipitated out of water or another fluid.
REVERSE FAULT: See THRUST FAULT.
RHYOLITE: A quartz-rich, felsic, fine-grained, and usually light-colored igneous rock. The extrusive equivalent of granite, it is at least 10% quartz.
RING DIKE: A dike, associated with volcanoes and calderas, which has a circular pattern as seen from above.
RIPRAP: Large rock fragments used to stabilize a shoreline or to help stabilize a slope or hillside.
ROAD METAL: See BALLAST.
ROCK: A consolidated assemblage of one or more types of minerals.
ROCKFALL: The form of mass wasting in which individual slabs of rock fall down a slope, often one at a time.
RODINIA: The supercontinent that included most modern continents and that existed from approximately 1.1 bya-750 mya.
RUBBLE: In general parlance, broken or rough-edged stone fragments. In architecture, the term often refers to quarried stone that has been cut with at least one rough or rugged face. Cf. ASHLAR.
SALINE: Containing salt--often the mineral halite specifically.
SAND: A rock or mineral particle .06-2 mm (.0024-.08 in) in diameter.
SANDSTONE: A clastic rock composed of sand grains that have been cemented together with silica, calcite, or another mineral.
SAUK SEQUENCE: The sequence dating from the Vendian Period to the early Ordovician Period.
SECONDARY MINERAL: A mineral present in parent materials or soils that have not yet been significantly weathered.
SEDIMENTARY ROCK: Layered rock formed either by clastic particles or by chemical precipitation. Examples include arkose, breccia, chert, conglomerate, dolostone, graywacke, limestone, marl, sandstone, and shale.
SERPENTINITE: An exotic metamorphic rock that is usually deep-green with snaking white veins. This rock is thought to originate near the Earth's crust-mantle boundary. It is subsequently scraped up and "plastered" onto continental crust by the forces of continental or island-arc collision.
SCHIST: A low-grade metamorphic rock often characterized by a wavy, banded texture.
SCREE: See TALUS.
SEQUENCE: A stratigraphic term referring to a large assemblage of marine sedimentary rocks bounded above and below by unconformities. Such a deposit represents a span of geologic time when sea level rose and saltwater seas covered a large portion of continental interiors.
SHALE: A clastic rock composed of clay-sized particles.
SHIELD VOLCANO: A broad but gentle-sloped volcano that usually produces only lava flows. Cf. COMPOSITE VOLCANO and STRATOVOLCANO.
SILICATE MINERAL: A member of a chemical group of minerals that contains, among other substances, -SiO4. The one exception, quartz, is SiO2.
SIALIC ROCK: A rock, such as granite or rhyolite, which is rich in silicon and aluminum. Cf. SIMATIC ROCK.
SILL: A horizontal or slanting body of intrusive rock that forms when upward-moving magma pushes its way between two strata.
SILT: A mineral particle .003-.06 mm (.0001- .003 in) in diameter.
SILTSTONE: A clastic, sedimentary rock composed of silt particles.
SILURIAN PERIOD: The span of geologic time from approximately 440-410 mya.
SIMATIC ROCK: A rock, such as gabbro or basalt, which is rich in silicon and magnesium. Cf. SIALIC ROCK.
SINKHOLE: A depression in the Earth's surface, usually circular or oval, that is caused either by the surface solution of carbonate rocks or by the collapse of an underground cavern.
SLATE: The low-grade metamorphic equivalent of shale.
SLIDE: A form of mass wasting in which Earth materials move downslope even though they have not been lubricated by a substantial amount of water.
SLUMP BLOCK: A section of a bedrock cliff that has become detached and that has come to rest below or in front of the cliff face.
SLUMPING: A form of mass wasting occurring on vertical or very steep cliffs, in which the falling material detaches itself in curving, wedgelike sections.
SMECTITE: A general term for secondary minerals that include such clays as montmorillonite. Smectites swell in the presence of water.
SODA STRAW: A small, fragile, and hollow stalactite.
SOIL: The living skin of the Earth, composed of three crucial components: mineral matter, humus, and living organisms. Cf. REGOLITH.
SOIL HORIZON: See HORIZON.
SOIL ORDER: See ORDER.
SOIL PROFILE: See PROFILE.
SPELEOTHEM: A catch-all term for any cave deposit formed by dripping or flowing water.
SPREADING CENTER: The boundary of two divergent plates, where the upwelling and eruption of magma creates new crust.
STALACTITE: A speleothem that hangs down from a cave ceiling without touching the floor. It is one form of dripstone.
STALAGMITE: A speleothem that extends up from a cave floor without reaching the ceiling. It is one form of dripstone.
STONE MOUNTAIN GRANITE: A gray granite quarried at Stone Mountain, Georgia. It is Pennsylvanian in age.
STRATIGRAPHY: The`subdiscipline of geology that is the study of the rock record. A main endeavor of stratigraphers is to identify, classify, and assign dates to different rock units.
STRATOVOLCANO: A steep-sided volcano that produces alternating lava flows and pyroclastic flows. Cf. COMPOSITE VOLCANO and SHIELD VOLCANO.
STRATUM (plural = STRATA): A layer of sedimentary rock.
STRIKE: The compass bearing of a horizonal line on a tilted stratum. A strike measurement is paired with a dip measurement to establish the three-dimensional orientation of the stratum.
STRIKE-SLIP FAULT: An inclined fracture in the crust in which the upper block (or hanging wall) slides sideways relative to the lower block (or footwall).
STROMATOLITE: A structure built in shallow water by successive layers of microorganisms, chiefly cyanobacteria, but also algae. In longitudinal section stromatolites have a laminated aspect; externally, they range from flat or wavy sheets to mushroom- or cone-shaped objects.
STROMBOLIAN ERUPTION: A series of periodic and relatively mild volcanic events that eject incandescent tephra.
SUBDUCTION: The process in which one plate sinks beneath another.
SUBSOIL: Usually considered synonymous with the B horizon of a soil profile, however it is sometimes to include all horizons (e. g., E, B and C) below the A horizon.
SULFATE MINERAL: A member of a chemical group of minerals that contain, among other substances, -SO4.
SULFIDE MINERAL: A member of a chemical group of minerals that contain, among other substances, -S (sulfur).
SUPERCONTINENT: A giant landmass formed by the collision of two or more continents.
SUPERIMPOSED STREAM: A stream that had its course defined by the original landscape through which it flowed. Subsequently, the stream cut down and exposed other geological structures that have no apparent relation to its course.
SUSPECT TERRANE: A terrane that is suspected of being, but not thoroughly demonstrated to be, allonchthonous.
SYENITE: A felsic and intrusive igneous rock similar in appearance to granite but with significantly less quartz content.
SYNCLINE: A structure in which the crust has been arched downward.
TALUS: Loose rock debris that collects at the base of a slope.
TEJAS SEQUENCE: The sequence dating from the late Paleogene Period to the present.
TEPHRA: A general term for any airborne particles or fragments ejected from a volcano, regardless of their size.
TERMINAL MORAINE: See END MORANE.
TENSIONAL FORCE: A force that pulls apart (stretches) a portion of the crust. Cf. COMPRESSIONAL FORCE.
TERRA COTTA: A fired and usually glazed clay used as architectural ornament.
TERRA ROSSA: A red or reddish-brown soil residue derived from the weathering of limestone.
TERRESTRIAL PLANET: A relatively small and rocky planet differentiated into crust, mantle, and metallic core. Our solar system's terrestrial planets include Mercury, Venus, Earth, and Mars.
TERRAIN: The surface expression of a landscape; the "lay of the land."
TERRANE: A three-dimensional section of the Earth's crust that is bounded by faults on all sides and that is composed of rock types that share a common origin.
TERTIARY PERIOD: The span of geologic time from approximately 65-2 mya. This older division of geologic time and the following Quaternary Period are gradually being supplanted by the system using the Paleogene Period and the Neogene Period instead.
THRUST FAULT: An inclined fracture in the crust in which the upper block (or hanging wall) slides up relative to the lower block (or footwall).
TILL: Unsorted rock rubble, ranging from boulders to clay particles, that is deposited under or directly in front of a glacier.
TIPPECANOE SEQUENCE: The sequence dating from the early Ordovician Period to the early Devonian Period.
TONALITE: An igneous intrusive rock resembling granite and of the same mineral composition as quartz diorite.
TOPSOIL: A soil's surface layer (A Horizon)
TRACE FOSSIL: A fossil that indicates some aspect of an ancient organism's behavior or locomotion, rather than some aspect of its form.
TRACHYANDESITE: See TRACHYTE.
TRACHYTE: A fine-grained and light- to dark-gray extrusive rock with a chemical composition intermediate between rhyolite and basalt.
TRANSFORM PLATE BOUNDARY: A zone, characterized by powerful earthquakes but no volcanic activity, where two plates move laterally past each other.
TRIASSIC PERIOD: The span of geologic time from approximately 245-208 mya.
TRILOBITE: An extinct marine arthropod animal that resembled a modern pillbug. Trilobites existed from almost the beginning of the Paleozoic Era to its end.
TRIPLE JUNCTION: The meeting point of three plates bounded by three rifts.
TUFF: Ash that has become lithified into a fine-grained volcanic rock.
TURBIDITY CURRENT: A dense current of suspended sediments, often triggered by marine earthquakes and landslides, which travels along the sea bottom from shallow water to deep water, and then deposits its sediments as a graded bed.
ULTISOL: An ancient, weathered soil order with a high clay content.
ULTRAMAFIC: Referring to rocks that are very rich in iron and magnesium. One such rock type is peridotite, which is thought to be the main constituent of the upper mantle.
UNCONFORMITY: A surface gap in the sedimentary record, as revealed in an outcrop.
UNCONSOLIDATED SEDIMENTS: All sediments that are loose and not part of the bedrock or of a larger, detached rock.
UNLITHIFED: Referring to loose sediments that have not been turned into rock.
UVALA: A karst landform in which two or more sinkholes have coalesced to form a larger depression.
VALLEY GLACIER: See ALPINE GLACIER.
VARANGIAN ICE AGE: The ice age, thought by some geologists to have been the severest, that occurred approximately 600 mya.
VENDIAN PERIOD: The span of geologic time from approximately 600-544 mya.
VENT: The opening through which lava and other volcanic materials reach the Earth's surface.
VERDE ANTIQUE: Another term for SERPENTINITE.
VERTISOL: A soil order characterized by the presence of deep, vertical cracks in the soil that penetrate the different horizons. Vertisols are found in climates where wet periods are followed by drought.
VESICLE: A small cavity in extrusive rock formed by a gas bubble when the rock was still molten or at least very hot.
VOLCANIC NECK: See NECK.
WACKESTONE: A type of carbonate rock in the Dunham Classification System with a grain content of 10-50% that is surrounded by a lime-mud matrix.
WEATHERING: The immensely powerful if often subtle process by which rock, soil, and other Earth materials experience a change in composition due to physical, chemical, or biologic factors acting on them.
WEIGHT-BEARING ELEMENT: See LOAD-BEARING ELEMENT.
WELDED TUFF: A tuff formed of ash particles that were still hot enough after they fell to the ground to fuse together.
XENOLITH: An older fragment of foreign rock contained within a younger igneous rock type.
ZUNI SEQUENCE: The sequence dating from the early Jurassic Period to the late Paleogene Period. It has been suggested that this sequence includes the highest sea level recorded in Earth history. That highstand occurred in the Cretaceous Period.
