Raymond Wiggers
Geology & Soil-Science Glossary
- Last Updated 21 January 2008 -
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.