aeolian--Pertaining to the wind, especially as it affects rocks, soils and deposits such as loess, dune sand and some volcanic tuffs the constituents of which were transported (blown) and laid down. Also pertains to sedimentary structures such as ripple made by wind or to geologic processes such as erosion and deposition accomplished by the wind.
albic horizon--A mineral soil horizon from which clay and free iron oxides have been removed or in which the oxides have been segregated to the extent that the color of the horizon is determined mostly by the color of the primary sand and silt particles rather than by the coatings on these particles.
Alfisols--The Order of mineral soils that has umbric or ochric epipedons, argillic horizons, and that holds water at less than 15 bars tension during at least three months when the soil is warm enough for plants to grow outdoors. Alfisols have a mean annual soil temperature of less than 8 C (46.4 F) or a base saturation in the part of the argillic horizon of 35 percent or more when measured at pH 8.2.
amphibole--A group of dark, rock-forming, ferromagnesian silicate minerals that are closely related in crystal form and composition. It is characterized by a cross-linked double chain of tetrahedra with a silicon:oxygen ratio of 4:11, by columnar or fibrous prismatic crystals and by good prismatic cleavage in two directions parallel to the crystal faces and intersecting at angles of about 56° and 124°. Amphibole varies in color from white to black and is an abundant and widely distributed constituent in igneous and metamorphic rocks.
anthropic epipedon--Said of an epipedon that is similar to a mollic epipedon but in which the content of soluble P2O5 is greater than 250 ppm. It develops due to long periods of cultivation and fertilization.
argillic horizon--A mineral soil horizon that is characterized by the illuvial accumulation of layer-lattice silicate clays. The argillic horizon has a certain minimum thickness depending on the thickness of the solum, a minimum quantity of clay in comparison with an overlying eluvial horizon depending on the clay content of the eluvial horizon, and usually has coatings of oriented clay on the surface of pores or peds or bridging sand grains.
augite--A term often used as a synonym of pyroxene. It may contain titanium and ferric iron. Augite usually is black, greenish black or dark green and commonly occurs as an essential constituent in many basic igneous rocks and in certain metamorphic rocks.
authigenic--Said of constituents and minerals that have not been transported or that were derived where they are now. Also said of minerals that came into existence at the same time, or subsequently to, the formation of the rock of which they constitute a part. The term, as used, often refers to a mineral such as quartz or feldspar formed after deposition of the original sediment.
badland--A land type generally devoid of vegetation and broken by an intricate maze of narrow ravines, sharp crests and pinnacles resulting from serious erosion of soft geologic materials. Most common in arid or semiarid regions.
biotite--A widely distributed and important rock-forming mineral of the mica group. It is generally black, dark brown or dark green and forms a constituent of crystalline rocks or a detrital constituent of sandstones and other sedimentary rocks. Also a general term to designate all ferromagnesian micas. Biotite is known also as black mica, iron mica and magnesia mica.
Boralfs--A suborder of Alfisols that has formed in cool places. Boralfs have frigid or cryic but not pergelic temperature regimes and have udic moisture regimes. Boralfs are not saturated with water for periods long enough to limit their use for most crops.
breccia--A coarse-grained clastic rock composed of large (greater than sand size, or 2 mm in diameter), angular and broken rock fragments cemented together in a finer-grained matrix that may or may not be similar to the larger fragments and that can be of any composition, origin or mode of accumulation; the consolidated equivalent of rubble. Breccia is similar to conglomerate except that most of the fragments have sharp edges and unworn corners; the term formerly included conglomerate and is still sometimes so used in Europe. The rock can be formed in many ways, but chiefly by sedimentation and igneous and tectonic activities.
calcic horizon--A mineral soil horizon of secondary carbonate enrichment that is more than 15 cm (6 in) thick, has a calcium carbonate equivalent of more than 15 percent and has at least 5 percent more calcium carbonate equivalent than the underlying C horizon.
cambic horizon--A mineral soil horizon that has a texture of loamy very fine sand or finer, has soil structure rather than rock structure, contains some weatherable minerals and is characterized by the alteration or removal of mineral material as indicated by mottling or gray colors, stronger chromas or redder hues than in underlying horizons, or the removal of carbonates. The cambic horizon lacks cementation or induration and has too few evidences of illuviation to meet the requirements of the argillic or spodic horizon.
cienega--A marshy area where the ground is wet due to the presence of seepage or springs, often with standing water and abundant vegetation. The term is commonly applied in arid regions such as the U.S. Southwest.
clastic--Pertaining to or being a rock or sediment composed principally of broken fragments that are derived from pre-existing rocks or minerals and that have been transported individually for some distance from their places of origin. Also said of the texture of such a rock. The term is often used to indicate a source from within the depositional basin, as compared with detrital. The commonest clastics are sandstones and shales.
cleft--An abrupt chasm, cut, breach or other sharp opening such as a craggy fissure in a rock, a wave-cut gully in a cliff, a trench in the ocean bottom, a notch in the rim of a volcanic crater or a narrow recess in a cave floor.
cliff--Any high, very steep to perpendicular or over-hanging face of rock (sometimes earth or ice) occurring in the mountains or rising above the shore of a lake or river; a precipice. A cliff is usually formed by erosion, less commonly by faulting.
clod--A compact, coherent mass of soil ranging in size from 0.5 to 1.0 cm (0.2 to 0.4 in) to as much as 20 to 25 cm (8 to 10 in). It is produced artificially, usually by the activity of man by plowing, digging, etc., especially when these operations are performed on soils that are either too wet or too dry for normal tillage operations.
cryic--A soil temperature regime that has mean annual soil temperatures of more than 0 C (32 F) but less than 8 C (46 F), more than 5 C (9 F) difference between mean summer and mean winter soil temperatures at 50 cm (20 in) and cold summer temperatures.
cutan--A modification of the texture, structure or fabric of soil material along a natural surface within it and caused by a concentration of a particular soil constituent. It can be composed of any of the component substances of the soil material.
detrital--Pertaining to or formed from detritus. Said especially of rocks, minerals and sediments. The term is often used to indicate a source from outside the depositional basin, as compared with clastic.
dioritic--Pertaining to a group of rocks intermediate in composition between acidic and basic rocks. They characteristically are composed of dark-colored amphibole, acid plagioclase, pyroxene and sometimes a small amount of quartz.
druse--An irregular cavity or opening in a vein or rock, having its interior surface or walls lined (encrusted) with small projecting crystals usually of the same minerals as those of the enclosing rock, and sometimes filled with water.
erosion--The wearing away of the land surface by running water, wind, ice or other geological agents, including such processes as gravitational creep, or detachment and movement of soil or rock by these processes.
escarpment--A long, more or less continuous cliff or relatively steep slope facing in one general direction. It breaks the general continuity of the land by separating two level or gently sloping surfaces. Erosion or faulting creates an escarpment.
exchangeable cation--A positively charged ion attached in available forms to clay and organic constituents of soils. Included are those of hydrogen and the alkali and alkaline earth metals, calcium, magnesium, potassium and sodium. These plant nutrients can be exchanged with each other and with other positively charged ions in soil solutions.
extragrade subgroup--That subgroup within a great group that has aberrant properties that do not represent intergrades to any known kind of soil identified in the hierarchical soil classification system. See intergrade subgroup and typic subgroup.
feldspar--A group of abundant rock-forming minerals that constitute 60 percent of the Earth's crust. They occur as components of all kinds of rocks and as fissure minerals in clefts and druse minerals in cavities. Feldspars are usually white or nearly white and clear and translucent, although they frequently are colored by impurities. On decomposition, feldspars yield a large part of the clay of soil and the mineral kaolinite.
filamentous actinomycetes--A non-taxonomic term applied to a threadlike group of organisms with characteristics intermediate between simple bacteria and true fungi. Most of these soil organisms are unicellular and produce a slender branched mycelium. They sporulate by segmentation of the entire mycelium, or more commonly by segmentation of special hyphae.
fissility--A general term for the property possessed by some rocks of splitting easily into thin sheets or layers along closely spaced, roughly planar, and approximately parallel surfaces, such as along bedding planes as in shale or along cleavage planes as in schist induced by fracture of flowage. Its presence distinguishes shale from mudstone. The term is not applied to minerals, but is analogous to cleavage in minerals.
foliate--Generally said of the planar arrangement of textural or structural features in any type of rock; e.g., cleavage in slate or schistosity in metamorphic rock. It is most commonly applied to a metamorphic rock.
fragipan--A natural subsurface horizon with high bulk density relative to the solum above. It is seemingly cemented when dry, but shows a moderate to weak brittleness when moist. The layer is low in organic matter, mottled, slowly or very slowly permeable to water, and usually shows occasional or frequent bleached cracks forming polygons. It may be found in profiles of either cultivated or virgin soils but not in calcareous material.
frigid soil--A soil temperature regime that has mean annual soil temperatures of more than 0 C (32 F) but less than 8 C (46 F), more than 5 C (9 F) difference between mean summer and mean winter soil temperatures at 50 cm (20 in), and warm summer temperatures. Isofrigid is the same except the summer and winter temperatures differ by less than 5 C (9 F).
gabbro--A group of dark-colored, basic intrusive igneous rocks composed principally of basic plagioclase and augite, with or without olivine and orthopyroxene. It is the approximate intrusive equivalent of basalt. Gabbro grades into monzonite with increasing alkali-feldspar content.
gilgai--The microrelief of soils produced by expansion and contraction with changes in moisture. Found in soils that contain large amounts of clay that swells and shrinks considerably with wetting and drying. Usually a succession of microbasins and microknolls in nearly level areas or of microvalleys and microridges parallel to the direction of the slope.
glaebules--A three-dimensional unit, usually prolate to equant in shape, within the matrix of a soil material. It is recognizable by a greater concentration of some constituent and by its difference in fabric as compared with the enclosing soil material . . . a small clod or lump of earth.
gneiss--A foliated rock formed by regional metamorphism in which bands or lenticles of granular minerals alternate with bands and lenticles in which minerals having flaky or elongate prismatic habits predominate. Generally, less than 50 percent of the minerals show the preferred parallel orientation. Although a gneiss is commonly feldspar- and quartz-rich, the mineral composition is not an essential factor in its definition. Varieties are distinguished by texture, characteristic minerals or general composition and/or origin.
graben--An elongate, relatively depressed crustal unit or block that is bounded by faults on its long sides. It is a structural form that may or may not be geomorphologically expressed as a rift valley.
granodiorite--A group of coarse-grained plutonic rocks intermediate in composition between quartz diorite and quartz monzonite containing quartz, plagioclase, and potassium feldspar, with biotite, hornblende, or, more rarely, pyroxene, as the mafic components. The ratio of plagioclase to total feldspar is at least two to one but less than nine to ten. With less alkali feldspar it grades into quartz diorite, and with more alkali feldspar, into granite or quartz monzonite.
gypsic horizon--A mineral soil horizon of secondary calcium sulfate enrichment that is more than 15 cm (6 in) thick, has at least 5 percent more gypsum than the C horizon, and in which the product of the thickness in centimeters and the percent calcium sulfate is equal to or greater than 150 percent cm.
histic epipedon--A thin organic soil horizon that is saturated with water at some period of the year unless artificially drained and that is at or near the surface of a mineral soil. The histic epipedon has a maximum thickness depending on the kind of materials in the horizon and the lower limit of organic carbon is the upper limit for the mollic epipedon.
Histosols--The Order of organic soils that has organic soil materials in more than half of the upper 80 cm (32 in), or that is of any thickness if overlying rock or fragmental materials that have interstices filled with organic soil materials.
hogback--Any ridge with a sharp summit and steep slopes of nearly equal inclination on both flanks, and resembling in outline the back of a hog. More specifically, a long, narrow, sharp-crested ridge formed by the outcropping edges of very steeply inclined or highly tilted resistant rocks such as igneous dikes and produced by differential erosion. The term is usually restricted to ridges carved from beds dipping at angles greater than 20 degrees.
hogwallow--A wallow made by swine. Also, a similar depression believed to be formed by heavy rains. Or, a faintly billowing land surface characterized by many low, coalescent or rounded mounds such as mima mounds that are slightly higher than the basin-shaped depressions between them.
hygroscopic water--Water adsorbed by a dry soil from an atmosphere of high relative humidity, water remaining in the soil after "air-drying" or water held by the soil when it is in equilibrium with an atmosphere of a specified relative humidity at a specified temperature, usually 98 percent relative humidity at 25 C (77 F).
Inceptisols--The Order of mineral soils that has one or more pedogenic horizons in which mineral materials other than carbonates or amorphous silica have been altered or removed but not accumulated to a significant degree. Under certain conditions, Inceptisols may have an ochric, umbric, histic, plaggen or mollic epipedon. Water is available to plants more than half of the year or more than three consecutive months during a warm season.
intergrade subgroup--The subgroup that contains soils of one great group but that has some properties characteristic of the soils of another great group or other class. These properties are not developed or expressed enough for the soils to be included within the second great group in the hierarchical soil classification system. (See extragrade subgroup and typic subgroup.)
kaolinite--A common clay mineral. A two-layer hydrous aluminum silicate having the general formula Al2(Si2O5)(OH)4. It consists of sheets of tetrahedrally coordinated silicon joined by an oxygen shared with octahedrally coordinated aluminium.
laccolithic--Pertains to a concordant igneous intrusion with a known or assumed flat floor and a postulated dikelike feeder somewhere beneath its thickest point. It is generally lenslike in form and roughly circular, less than 8 km (5 mi) in diameter, and from a meter or so to a few hundred meters in thickness.
latite--A porphyritic extrusive rock having plagioclase and potassium feldspar present in nearly equal amounts as phenocrysts, little or no quartz and a finely crystalline to glassy groundmass that may contain obscure potassium feldspar; the extrusive equivalent of monzonite.
leaching--The removal in solution of soil constituents such as mineral salts and organic matter from an upper to a lower soil horizon by the action of percolating water, either naturally by rainwater or artificially by irrigation.
listric surface--A curvilinear, usually concave-upward surface of fracture that curves at first gently and then more steeply from a horizontal position. Listric surfaces form wedge-shaped masses that appear to be thrust against or along each other.
Lithosols--The Order of azonal soils characterized by an incomplete solum or no clearly expressed soil morphology and consisting of freshly and imperfectly weathered rock or rock fragments. Lithosols usually develop on steep slopes.
loess--Material transported and deposited by wind and consisting mostly of silt-sized particles, including those of quartz, feldspar and carbonates. Loess and loess-derived soils are generally fertile.
mesic--A soil temperature regime that has mean annual soil temperatures of 8 C (46 F) or more but less than 15 C (59 F), and more than 5 C (9 F) difference between mean summer and mean winter soil temperatures at 50 cm (20 in). Isomesic is the same except the summer and winter temperatures differ by less than 5 C (9 F).
metamorphic rocks--Rocks derived from pre-existing rocks but that differ from them in physical, chemical and mineralogical properties as a result of natural geological processes, principally heat and pressure. The pre-existing rocks may have been igneous, sedimentary or another metamorphic rock.
mica (illite)--A mineral that consists of complex phyllosilicates with sheet-like structures that crystallize in forms apparently orthorhombic or hexagonal but really monoclinic. These structures are characterized by low hardness and by perfect basal cleavage, readily spitting into very thin, tough and somewhat elastic plates that have a splendent pearly luster on their surfaces. They commonly occur as flakes, scales or shreds and vary in color from colorless, silvery white, pale brown, or yellow to green or black. Micas are prominent rock-forming constituents of many igneous and metamorphic rocks.
mima mound--A term used in the U.S. Northwest to identify low, circular or oval domes composed of loose, unstratified gravelly silt and soil material built upon glacial outwash on a hog-wallow landscape. The basal diameter varies from 3 m (10 ft) to more than 30 m (100 ft) and the height from 30 cm (12 in) to about 2 m (7 ft). The mounds probably are built by pocket gophers.
mollic horizon--Pertains to a mineral soil horizon that is dark colored and relatively thick, contains at least 0.5 percent organic carbon, is not massive and hard or very hard when dry, has a base saturation of more than 50 percent when measured at pH 7, has less than 250 ppm of P2O5 soluble in 1 percent citric acid and is dominantly saturated with bivalent cations.
Mollisols--The Order of mineral soils that has a mollic epipedon overlying mineral material with a base saturation of 50 percent or more when measured at pH 7. Mollisols may have a horizon that is argillic, natric, albic, cambic, gypsic, calcic or petrocalcic, a histic epipedon or a duripan, but not an oxic or a spodic horizon.
montmorillonite--A crystalline clay mineral with a 2:1 expansible layer structure; that is, with two silicon tetrahedral sheets enclosing an aluminum octahedral sheet. It swells considerably with wetting and shrinks considerably when drying.
monzonite--A group of plutonic rocks intermediate in composition between syenite and diorite, containing approximately equal amounts of orthoclase and plagioclase, little or no quartz and commonly augite as the main mafic mineral. With decrease in the alkali feldspar content, monzonite grades into diorite or grabbo, depending on the composition of the plagioclase. With an increase in alkali feldspar, it grades into syenite.
mor--A type of forest humus in which an Oa horizon is present and in which practically no mixing of surface organic matter with mineral soil occurs. The transition from the O layer to A horizon is abrupt.
mull--A type of forest humus in which an Oe horizon may or may not be present and in which no Oa horizon exists. The A horizon consists of an intimate mixture of organic matter and mineral soil. The transition between the A and the horizon beneath is gradual.
natric horizon--A mineral soil horizon that satisfies the requirements of an argillic horizon, but that also has a prismatic, columnar or blocky structure and a subhorizon that has more than 15 percent saturation with exchangeable sodium.
nickpoint (also knickpoint)--Any interruption or break of slope. More specifically, it is a point of abrupt change or inflection in the longitudinal profile of a stream or of its valley that occurs where a new curve or erosion (graded to a new base level after a relative lowering of the former level) intersects an earlier curve. A nickpoint results from rejuvenation, glacial erosion or the outcropping of a resistant bed.
ochric horizon--Pertains to a surface horizon of mineral soil that is too light in color, too high in chroma, too low in organic carbon or too thin to be an epipedon that is plaggen, mollic, umbric, anthropic or histic, or that is both hard and massive when dry.
organic soils--Soils that are saturated with water and have 17.4 percent or more organic carbon if the mineral fraction is 50 percent or more clay, or 11.6 percent organic carbon if the mineral fraction has no clay, or has proportional intermediate contents, or if never saturated with water, have 20.3 percent or more organic carbon.
orogeny--The process by which structures within mountain areas were formed, including thrusting, folding and faulting in the outer and higher layers, and plastic folding, metamorphism and plutonism in the inner and deeper layers.
orthoclase--A colorless, white, cream-yellow, flesh-reddish, or grayish mineral of the alkali-feldspar group. It usually contains some sodium in minor amounts. Ordinary or common orthoclase is one of the commonest rock-forming minerals. It occurs especially in granites, acid igneous rocks and crystalline schists.
oxic horizon--A mineral soil horizon that is at least 30 cm (12 in) thick and characterized by the virtual absence of weatherable primary minerals or 2:1 lattice clays, the presence of 1:1 lattice clays and highly insoluble minerals such as quartz sand, the presence of hydrated oxides of iron and aluminum, the absence of water-dispersible clay, and the presence of low cation exchange capacity and small amounts of exchangeable bases.
paleo---A combining form denoting the attribute of great age or remoteness in regard to time, or involving ancient conditions, or of ancestral origin or dealing with fossil forms. Also a prefix indicating pre-Tertiary origin, and origin and generally altered character of a rock to the name of which it is added.
pediment--A broad, flat or gently sloping rock-floored erosion surface or plain of low relief. It typically is developed by subaerial agents including running water in an arid or semiarid region at the base of an abrupt and receding mountain front or plateau escarpment. Bedrock and occasionally older alluvial deposits that may be varve underlie the pediment. It is often partly mantled with a thin, discontinuous veneer of alluvium derived from the upland masses and in transit across the surface. The longitudinal profile of a pediment is normally slightly concave upward. Its outward form may resemble a piedmont slope, which continues the forward inclination of a pediment.
pedon--The smallest unit or volume of soil that represents or exemplifies all the horizons of a soil profile. It is usually a horizontal, more or less hexagonal area of about one square meter, but may be larger.
periglacial--Said of the processes, conditions, areas, climates and topography at the immediate margins of former and existing glaciers and ice sheets, and influenced by the cold temperature of the ice. By extension, said of an environment in which frost action is an important factor, or of phenomena induced by a periglacial climate beyond the periphery of the ice.
petrocalcic horizon--A continuous, indurated calcic horizon that is cemented by calcium carbonate and in some places with magnesium carbonate. It cannot be penetrated with a spade or auger when dry, dry fragments do not slake in water and it is impenetrable to roots.
phyllosilicate--A class or structural type of silicate characterized by the sharing of three of the four oxygens in each tetrahedron with neighboring tetrahedra to form flat sheets. The silicon to oxygen ratio is 2:5. Micas are an example of phyllosilicate.
pluvial--Said of a geologic episode, change, process, deposit or feature resulting from the action or effects of rain. For example, pluvial denudation, a landslide, or gully erosion and the consequent spreading out of the eroded material below. The term sometimes includes the fluvial action of rainwater flowing in a stream channel, especially in the channel of an ephemeral stream.
porphyritic--Said of the texture of an igneous rock in which larger crystals (phenocrysts) are set in a finer groundmass that may be crystalline or glassy or both. Also, said of a rock with such texture or of the mineral forming the phenocrysts. The term is sometimes restricted to cases in which the phenocrysts and groundmass formed during two different crystallization generations.
pyroclastic--Pertaining to clastic rock material formed by volcanic explosion or aerial expulsion from a volcanic vent; also, pertaining to rock texture of explosive origin. It is not synonymous with the adjective "volcanic."
pyroxene--A group of dark, rock-forming silicate minerals closely related in crystal form and composition. Pyroxenes are a common constituent of igneous rocks and are analogous in chemical composition to the amphiboles, except that they lack hydroxyls.
quartz--Crystalline silica, an important rock-forming mineral. It is, next to feldspar, the commonest mineral. It occurs either in colorless and transparent hexagonal crystals (sometimes colored yellow, brown, purple, red, green, blue or black by impurities) or in crystalline or cryptocrystalline masses. Quartz is the commonest gangue mineral of ore deposits, forms the major proportion of most sands and has widespread distribution in igneous, metamorphic and sedimentary rocks.
radiometric dating--Calculating an age in years for geologic materials by measuring the presence of a short-life radioactive element, such as carbon-14. Calculations are made also by measuring the presence of a long-life radioactive element plus its decay product, such as potassium-40/argon-40. The term applies to all methods of age determination based on nuclear decay of natural elements.
rhyodacite--A group of extrusive porphyritic igneous rocks intermediate in composition between dacite and rhyolite. Quartz, plagioclase and biotite are the main phenocryst minerals. These rocks have a fine-grained to glassy groundmass composed of alkali feldspar and silica minerals. Rhyodacite is the extrusive equivalent of granodiorite or quartz monzonite.
rhyolitic--Pertains to a group of extrusive igneous rocks. They are generally porphyritic and exhibit flow texture. These rocks have phenocrysts of quartz and alkali feldspar in a glassy to crytocrystalline groundmass. Rhyolite grades into rhyodacite with decreasing alkali feldspar content.
salic horizon--A mineral soil horizon of enrichment with secondary salts more soluble than gypsum in cold water. A salic horizon is 15 cm (6 in) or more in thickness and contains at least 2 percent salt. The product of the thickness in centimeters and percent salt by weight is 60 percent cm or more.
saprolite--A soft, earthy, clay-rich, thoroughly decomposed rock formed in place by chemical weathering of igneous and metamorphic rocks. It often forms a thick, as much as 100 m (330 ft), layer or cover, especially in a humid and tropical or subtropical climate. The color is commonly some shade of red or brown.
schist--A strongly foliated crystalline rock formed by dynamic metamorphism that can be readily split into thin flakes or slabs due to the well-developed parallelism of more than 50 percent of the minerals present, particularly those of lamellar or elongate prismatic habit.
schistose--Said of a rock displaying schistosity, the foliation in schist or other coarse-grained, crystalline rock due to the parallel, planar arrangement of mineral grains of the platy, prismatic or ellipsoidal types, usually mica.
sclerenchyma--A tissue in higher plants composed of cells that have thickened and lignified cell walls and often mineralized. They are usually without living protoplasm and incapable of further growth when mature. Sclerenchyma is supporting or protective tissue. Its cells are of two general types, elongated, pointed cells and shorter cells of irregular size and shape.
sedimentary rock--Rock formed from materials deposited from suspension or precipitated from solution and usually more or less consolidated. The principal sedimentary rocks are sandstones, shales, limestones and conglomerates.
silicic--Said of a silica-rich igneous rock or magma. The amount of silica usually constitutes about 65 percent of the rock. In addition to the combined silica in feldspars, silicic rocks generally contain free silica in the form of quartz. Granite and rhyolite are typical silicic rocks.
skeleton grains--Relatively stable and not readily translocated grains of soil material, concentrated or reorganized by soil-forming processes, e.g., a mineral grain, or a resistant siliceous or organic body larger than colloidal size.
slope of soil--The slope of soil is reported as gradient, i.e., the number of meters (feet) of rise or fall per 30 m (100 ft) horizontal distance. If the soil surface rises 3 m (10 ft) vertically over a horizontal distance of 30 m (100 ft) the slope gradient is 10 percent.
soil association--A group of defined and named taxonomic soil units occurring together in an individual and characteristic pattern over a geographic region. It is comparable with plant associations in many ways. The term also describes a mapping unit used on general soil maps in which two or more defined taxonomic units occurring together in a characteristic pattern are combined because the scale of the map or the purpose for which it is being made does not require delineation of the individual soils.
soil complex--A mapping unit used in detailed soil surveys where two or more defined taxonomic units are so intimately intermixed geographically that it is undesirable or impractical to separate them because of the scale being used. A more intimate mixing of smaller areas of individual taxonomic units than that described under soil association.
soil drainage--Natural soil drainage refers to the speed with which water is removed from the soil surface and through the soil itself. Seven classes are recognized: excessive, somewhat excessive, well, moderately well, somewhat poor (imperfect), poor and very poor. Artificial drainage refers to removal of water by ditching, tilling and construction of surface waterways and terraces.
soil genesis--The mode of the origin of the soil, with special reference to the processes of soil-forming factors responsible for the development of the solum, or true soil, from unconsolidated parent material.
soil horizon--A layer of soil or soil material approximately parallel to the land surface and differing from adjacent genetically related layers in physical, chemical and biological properties or characteristics such as color, structure, texture, consistency, kinds and numbers of organisms present, and degree of acidity or alkalinity. See the Soil Horizons section of the first chapter.
soil morphology--The physical constitution of a soil profile as exhibited by the kinds, thickness and arrangement of the horizons in the profile and by the texture, structure, consistency and porosity of each horizon. Also, the structural characteristics of the soil or any of its parts.
soil series--The basic unit of soil classification. A subdivision of a soil family that consists of soils that are essentially alike in all major profile characteristics except the texture of the A horizon.
soil structure--The combination or arrangement of primary soil particles into secondary particles, units or peds. These secondary units may be arranged in the profile in such a manner as to give a distinctive characteristic pattern, but usually are not. The secondary units are characterized and classified on the basis of size, shape and degree of distinctness into classes, types and grades, respectively. See soil structure classes, soil structure grades and soil structure types.
soil structure grades--A grouping or classification of soil structure on the basis of inter- and intra-aggregate adhesion, cohesion or stability within the profile. Four grades of structure designated from 0 to 3 are described below.
soil texture--The relative proportions of the various soil separates in a soil. The textural classes may be modified by the addition of suitable adjectives when coarse fragments are present in substantial amounts; for example, "stony silt loam," or "silt loam, stony phase." The sand, loamy sand and sandy loam are further subdivided on the basis of the proportions of the various sand separates present. The limits of the various classes and subclasses are defined below.
spodic horizon--A mineral soil horizon that is characterized by the illuvial accumulation of amorphous materials composed of aluminum and organic carbon with or without iron. The spodic horizon has a certain minimum thickness, and a minimum quantity of extractable carbon plus iron plus aluminum in relation to its content of clay.
syenite--A group of plutonic rocks containing alkali feldspar, a small amount of plagioclase (less than in monzonite), one or more mafic minerals and quartz, if present, only as an accessory. With an increase in the quartz content, syenite grades into granite. Its name is derived from Syene,
tectonics, plate--Global tectonics based on an Earth model characterized by a small number of large, broad, thick plates that float on viscous underlayers in the mantle. The plates move more or less independently and grind against others like ice floes in a river. Much of the dynamic activity is concentrated along the periphery of the plates, which are propelled from the rear by seafloor spreading. The continents form a part of the plates and move with them, like logs frozen in ice floes.
typic subgroup--That subgroup that represents the central concept of the soils classified in a great group within the hierarchical soil classification system. (See extragrade subgroup and intergrade subgroup.)
udic--A soil moisture regime that is neither dry for as long as 90 cumulative days nor for as long as 60 consecutive days in the 90 days following the summer solstice at periods when the soil temperature at 50 cm (20 in) is above 5 C (41 F).
umbric epipedon--A surface layer of mineral soil that has the same requirements as the mollic epipedon with respect to color, thickness, organic carbon content, consistence, structure and P2O5 content, but that has a base saturation of less than 50 percent when measured at pH 7.
varve--A distinct band representing the annual deposit in sedimentary materials regardless of origin and usually consisting of two layers, one a thick, light-colored layer of silt and fine sand and the other a thin, dark-colored layer of clay.
vermiculite--A group of platy or micaceous clay minerals closely related to chlorite and montmorillonite. The minerals are derived generally from the alteration of micas and they vary widely in chemical composition. They are characterized by marked exfoliation when heated above 150 C (302 F). At high temperature, their granules greatly expand into long, worm-like threads that entrap air and produce a lightweight and high water-absorbent material that is used as an insulator and as an aggregate in concrete and plaster.
Vertisols--The Order of mineral soils that has 30 percent or more clay, deep, wide cracks when dry and gilgai microrelief, intersecting slickensides or wedgeshaped structural aggregates tilted at an angle from the horizontal.