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Soils Glossary
August 14, 2010
AGE. Elapsed time in calendar years. Because the cosmic production of C-14 has varied during the Quaternary, radiocarbon years (expressed as ky B.P.) must be corrected by using tree-ring and other data. Abbreviations used for corrected ages are: ka (kilo anno or years in thousands) or Ma (millions of years). Abbreviations used for intervals are: yr (years), ky (thousands of years). radiocarbon ages = yr B.P. Calibrated ages are calculated from process assumptions, relative ages fit in a sequence, and correlated ages refer to matching unit. (See also yr B.P., HOLOCENE, PLEISTOCENE, QUATERNARY, PEDOCHRONOLOGY).
AGGRADATION. A modification of the earth's surface in
the direction of uniformity of grade by deposition.
ALKALI (SODIC) SOIL.
A soil having so high a degree of alkalinity (pH 8.5 or
higher), or so high a percentage of exchangeable sodium (15 % or more of the
total exchangeable bases), or both, that plant growth is restricted.
ALKALINE SOIL. Any soil that has a pH greater than 7.3.
(See Reaction, Soil.)
ANGULAR ORPHANS. Angular fragments separated from
weathered, well-rounded cobbles in colluvium derived from conglomerate.
ARGILLAN. (See Clay Film.)
ARGILLIC HORIZON. A horizon containing clay either
translocated from above or formed in place through pedogenesis.
ALLUVIATION. The process of building up of sediments by
a stream at places where stream velocity is decreased. The coarsest particles
settle first and the finest particles settle last.
ANOXIC. (See also GLEYED SOIL). A soil having a low
redox potential.
AQUICLUDE. A saturated body of sediment or rock that is
incapable of transmitting significant quantities of water under ordinary
hydraulic gradients.
aquitard.
A body of rock or sediment that retards but does not
prevent the flow of water to or from an adjacent aquifer. It does not readily
yield water to wells or springs but may serve as a storage unit for groundwater.
ATTERBERG LIMITS. The moisture content at which a soil
passes from a semi-solid to a plastic state (plastic limit, PL) and from a
plastic to a liquid state (liquid limit, LL). The plasticity index (PI) is the
numerical difference between the LL and the PL.
BEDROCK. The solid rock that underlies the soil and
other unconsolidated material or that is exposed at the surface.
BISEQUUM. Two soils in vertical sequence, each soil
containing an eluvial horizon and its underlying B horizon.
Boudin, boudinage. From a French word for sausage, describes the way that
layers of rock break up under extension. Imagine the hand, fingers together,
flat on the table, encased in soft clay and being squeezed from above, as being
like a layer of rock. As the spreading clay moves the fingers (sausages)
apart, the most mobile rock fractions are drawn or squeezed into the developing
gaps.
BURIED SOIL. A developed soil that was once exposed but
is now overlain by a more recently formed soil.
CALCAREOUS SOIL. A soil containing enough calcium
carbonate (commonly with magnesium carbonate) to effervesce (fizz) visibly when
treated with cold, dilute hydrochloric acid. A soil having measurable amounts of
calcium carbonate or magnesium carbonate.
CATENA. A sequence of soils of about the same age,
derived from similar parent material and occurring under similar climatic
conditions, but having different characteristics due to variation in relief and
drainage. (See also TOPOSEQUENCE.)
CEC. Cation exchange capacity. The amount of negative
charge balanced by positively charged ions (cations) that are exchangeable by
other cations in solution (meq/100 g soil = cmol(+)/kg soil).
CLAY. As a soil separate, the mineral soil particles
are less than 0.002 mm in diameter. As a soil textural class, soil material that
is 40 percent or more clay, less than 45 percent sand, and less than 40 percent
silt.
CLAY FILM. A coating of oriented clay on the surface of
a sand grain, pebble, soil aggregate, or ped. Clay films also line pores or root
channels and bridge sand grains. Frequency classification is based on the
percent of the ped faces and/or pores that contain films: very few--<5%;
few--5-25%; common--25-50%; many--50-90%; and continuous--90-100%. Thickness
classification is based on visibility of sand grains: thin--very fine sand
grains standout; moderately thick--very fine sand grains impart microrelief to
film; thick--fine sand grains enveloped by clay and films visible without
magnification. Synonyms: clay skin, clay coat, argillan, illuviation cutan.
COBBLE. Rounded or partially rounded fragments of rock
ranging from 7.5 to 25 cm in diameter.
COLLUVIUM. Any loose mass of soil or rock fragments
that moves downslope largely by the force of gravity. Usually it is thicker at
the base of the slope.
COLLUVIUM-FILLED SWALE. The prefailure topography of
the source area of a debris flow.
COMPARATIVE PEDOLOGY. The comparison of soils,
particularly through examination of features known to evolve through time.
CONCRETIONS. Grains, pellets, or nodules of various
sizes, shapes, and colors consisting of concentrated compounds or cemented soil
grains. The composition of most concretions is unlike that of the surrounding
soil. Calcium carbonate and iron oxide are common compounds in concretions.
CONDUCTIVITY. The ability of a soil solution to conduct
electricity, generally expressed as the reciprocal of the electrical
resistivity. Electrical conductance is the reciprocal of the resistance (1/R =
1/ohm = ohm-1 = mho [reverse of ohm] = siemens = S), while electrical
conductivity is the reciprocal of the electrical resistivity (EC = 1/r =
1/ohm-cm = mho/cm = S/cm or mmho/cm = dS/m). EC, expressed as uS/cm, is
equivalent to the ppm of salt in solution when multiplied by 0.640. Pure rain
water has an EC of 0, standard 0.01 N KCl is 1411.8 uS at 25C, and the
growth of salt-sensitive crops is restricted in soils having saturation extracts
with an EC greater than 2,000 uS/cm. Measurements in soils are usually performed
on 1:1 suspensions containing one part by weight of soil and one part by weight
of distilled water.
CONSISTENCE, SOIL. The feel of the soil and the ease
with which a lump can be crushed by the fingers. Terms commonly used to describe
consistence are --
Loose.--Noncoherent when dry or moist; does not hold
together in a mass.
Friable.--When moist, crushes easily under gentle
pressure between thumb and forefinger and can be pressed together into a lump.
Firm.--When moist, crushes under moderate pressure
between thumb and forefinger, but resistance is distinctly noticeable.
Plastic.--When wet, readily deformed by moderate
pressure but can be pressed into a lump; will form a "wire" when rolled between
thumb and forefinger.
Sticky.--When wet, adheres to other material, and tends
to stretch somewhat and pull apart, rather than to pull free from other
material.
Hard.--When dry, moderately resistant to pressure; can
be broken with difficulty between thumb and forefinger.
Soft.--When dry, breaks into powder or individual
grains under very slight pressure.
Cemented.--Hard and brittle; little affected by
moistening.
CTPOT. Easily remembered acronym for climate,
topography, parent material, organisms, and time; the five factors of soil
formation.
CUMULIC. A soil horizon that has undergone aggradation
coincident with its active development.
CUTAN. (See Clay Film.)
DEBRIS FLOW. Incoherent or broken masses of rock, soil,
and other debris that move downslope in a manner similar to a viscous fluid.
DEBRIS SLOPE. A constant slope with debris on it from
the free face above.
DEGRADATION. A modification of the earth's surface by
erosion.
DURIPAN. A subsurface soil horizon that is cemented by
illuvial silica, generally deposited as opal or microcrystalline silica, to the
degree that less than 50 percent of the volume of air-dry fragments will slake
in water or HCl.
ELUVIATION. The removal of soluble material and solid
particles, mostly clay and humus, from a soil horizon by percolating water.
EOLIAN. Deposits laid down by the wind, landforms eroded by the wind, or
structures such as ripple marks made by the wind.
FAULT-LINE SCARP. A scarp that has been produced by
differential erosion along an old fault line.
FAULTSLIDE. A landslide that shows physical evidence of
its interaction with a fault.
FIRST-ORDER DRAINAGE. The most upstream,
field-discernible concavity that conducts water and sediments to lower parts of
a watershed.
FLOOD PLAIN. A nearly level alluvial plain that borders
a stream and is subject to flooding unless protected artificially.
FOSSIL FISSURE. A buried rectilinear chamber associated
with extension due to ground movement. The chamber must be oriented along the
strike of the shear and must have vertical and horizontal dimensions greater
than its width. It must show no evidence of faunal activity and its walls may
have silt or clay coatings indicative of frequent temporary saturation with
ground water. May be mistaken for an animal burrow. Also known as a
paleofissure.
FRIABILITY. Term for the ease with which soil crumbles.
A friable soil is one that crumbles easily.
GENESIS, SOIL. The mode of origin of the soil. Refers
especially to the processes or soil-forming factors responsible for the
formation of the solum (A and B horizons) from the unconsolidated parent
material.
GEOMORPHIC. Pertaining to the form of the surface
features of the earth. Specifically, geomorphology is the analysis of landforms
and their mode of origin.
GLEYED SOIL. A soil having one or more neutral gray
horizons as a result of water logging and lack of oxygen. The term "gleyed" also
designates gray horizons and horizons having yellow and gray mottles as a result
of intermittent water logging.
GRAVEL. Rounded or angular fragments of rock 2 to 75 mm
in diameter. Soil textures with >15% gravel have the prefix "gravelly" and those
with >90% gravel have the suffix "gravel."
HIGHSTAND. The highest elevation reached by the ocean
during an interglacial period.
HOLOCENE. The most recent epoch of geologic time,
extending from 10 ka to the present.
HORIZON, SOIL. A layer of soil, approximately parallel
to the surface, that has distinct characteristics produced by soil-forming
processes. These are the major soil horizons:
O horizon.--The layer of organic matter on the surface
of a mineral soil. This layer consists of decaying plant residues.
A horizon.--The mineral horizon at the surface or just
below an O horizon. This horizon is the one in which living organisms are most
active and therefore is marked by the accumulation of humus. The horizon may
have lost one or more of soluble salts, clay, and sesquioxides (iron and
aluminum oxides).
E horizon -- This eluvial horizon is light in color,
lying beneath the A horizon and above the B horizon. It is made up mostly of
sand and silt, having lost most of its clay and iron oxides through reduction,
chelation, and translocation.
B horizon.--The mineral horizon below an A horizon. The
B horizon is in part a layer of change from the overlying A to the underlying C
horizon. The B horizon also has distinctive characteristics caused (1) by
accumulation of clay, sesquioxides, humus, or some combination of these; (2) by
prismatic or blocky structure; (3) by redder or stronger colors than the A
horizon; or (4) by some combination of these.
C horizon.--The relatively unweathered material
immediately beneath the solum. Included are sediment, saprolite, organic matter,
and bedrock excavatable with a spade. In most soils this material is presumed to
be like that from which the overlying horizons were formed. If the material is
known to be different from that in the solum, a number precedes the letter C.
R layer.--Consolidated rock not excavatable with a
spade. It may contain a few cracks filled with roots or clay or oxides. The rock
usually underlies a C horizon but may be immediately beneath an A or B horizon.
Major horizons may be further distinguished by applying
prefix Arabic numbers to designate differences in parent materials as they are
encountered (e.g., 2B, 2BC, 3C) or by applying suffix numerals to designate
minor changes (e.g., B1, B2).
Thereafter, these lower-case letters may be appended
(e.g., 2B2tkb):
a Highly decomposed organic
material
This symbol is used with O to indicate the most highly decomposed organic
materials, which have a fiber content of less than 17 percent (by volume) after
rubbing.
b Buried genetic
horizon
This symbol is used in mineral soils to indicate identifiable buried
horizons with major genetic features that were developed before burial. Genetic
horizons may or may not have formed in the overlying material, which may be
either like or unlike the assumed parent material of the buried soil. This
symbol is not used in organic soils, nor is it used to separate an organic layer
from a mineral layer.
c Concretions or
nodules
This symbol indicates a significant accumulation of concretions or
nodules. Cementation is required. The cementing agent commonly is iron,
aluminum, manganese, or titanium. It cannot be silica, dolomite, calcite, or
more soluble salts.
d Physical root
restriction
This symbol indicates noncemented, root-restricting layers in naturally
occurring or human-made sediments or materials. Examples are dense basal till,
plowpans, and other mechanically compacted zones.
e Organic material of
intermediate decomposition
This symbol is used with O to indicate organic materials of intermediate
decomposition. The fiber content of these materials is 17 to 40 percent (by
volume) after rubbing.
f Frozen soil or
water
This symbol indicates that a horizon or layer contains permanent ice. The
symbol is not used for seasonally frozen layers or for dry permafrost.
ff Dry
permafrost
This symbol indicates a horizon or layer that is continually colder than
0 oC and does not contain enough ice to be cemented by ice. This
suffix is not used for horizons or layers that have a temperature warmer than 0
oC at some time of the year.
g Strong gleying
This symbol indicates either that iron has been reduced and removed
during soil formation or that saturation with stagnant water has preserved it in
a reduced state. Most of the affected layers have chroma of 2 or less, and many
have redox concentrations. The low chroma can represent either the color of
reduced iron or the color of uncoated sand and silt particles from which iron
has been removed. The symbol g is not used for materials of low chroma that have
no history of wetness, such as some shales or E horizons. If g is used with B,
pedogenic change in addition to gleying is implied. If no other pedogenic change
besides gleying has taken place, the horizon is designated Cg.
h Illuvial
accumulation of organic matter
This symbol is used with B to indicate the accumulation of illuvial,
amorphous, dispersible complexes of organic matter and sesquioxides if the
sesquioxide component is dominated by aluminum but is present only in very small
quantities. The organosesquioxide material coats sand and silt particles. In
some horizons these coatings have coalesced, filled pores, and cemented the
horizon. The symbol h is also used in combination with s as “Bhs” if the amount
of the sesquioxide component is significant but the color value and chroma,
moist, of the horizon are 3 or less.
i Slightly
decomposed organic material
This symbol is used with O to indicate the least decomposed of the
organic materials. The fiber content of these materials is 40 percent or more
(by volume) after rubbing.
j Accumulation
of jarosite
Jarosite is a potassium or iron sulfate mineral that is commonly an
alteration product of pyrite that has been exposed to an oxidizing environment.
Jarosite has hue of 2.5Y or yellower and normally has chroma of 6 or more,
although chromas as low as 3 or 4 have been reported.
jj Evidence of
cryoturbation
Evidence of cryoturbation includes irregular and broken horizon
boundaries, sorted rock fragments, and organic soil materials occurring as
bodies and broken layers within and/or between mineral soil layers. The organic
bodies and layers are most commonly at the contact between the active layer and
the permafrost.
k Accumulation
of carbonates
This symbol indicates an accumulation of alkaline earth carbonates,
commonly calcium carbonate.
l Undefined as
of 2006.
If used, it
could be confused with the Arabic number “1”.
m Cementation or induration
This symbol indicates continuous or nearly continuous cementation. It is
used only for horizons that are more than 90 percent cemented, although they may
be fractured. The cemented layer is physically root restrictive. The predominant
cementing agent (or the two dominant cementing agents) may be indicated by
adding defined letter suffixes, singly or in pairs. The horizon suffix km
indicates cementation by carbonates; qm, cementation by silica; sm, cementation
by iron; ym, cementation by gypsum; kqm, cementation by lime and silica; and zm,
cementation by salts more soluble than gypsum.
n Accumulation of
sodium
This symbol indicates an accumulation of exchangeable sodium.
o Residual
accumulation of sesquioxides
This symbol indicates a residual accumulation of sesquioxides.
p Tillage or
other disturbance
This symbol indicates a disturbance of the surface layer by mechanical
means, pasturing, or similar uses. A disturbed organic horizon is designated Op.
A disturbed mineral horizon is designated Ap even though it is clearly a former
E, B, or C horizon.
q Accumulation
of silica
This symbol indicates an accumulation of secondary silica.
r Weathered or
soft bedrock
This symbol is used with C to indicate cemented layers (moderately
cemented or less cemented). Examples are weathered igneous rock and partly
consolidated sandstone, siltstone, or shale. The excavation difficulty is low to
high.
s Illuvial
accumulation of sesquioxides and organic matter
This symbol is used with B to indicate an accumulation of illuvial,
amorphous, dispersible complexes of organic matter and sesquioxides if both the
organic-matter and sesquioxide components are significant and if either the
color value or chroma, moist, of the horizon is 4 or more. The symbol is also
used in combination with h as “Bhs” if both the organic-matter and sesquioxide
components are significant and if the color value and chroma, moist, are 3 or
less.
ss Presence of
slickensides
This symbol indicates the presence of slickensides. Slickensides result
directly from the swelling of clay minerals and shear failure, commonly at
angles of 20 to 60 degrees above horizontal. They are indicators that other
vertic characteristics, such as wedge-shaped peds and surface cracks, may be
present.
t Accumulation
of silicate clay
This symbol indicates an accumulation of silicate clay that either has
formed within a horizon and subsequently has been translocated within the
horizon or has been moved into the horizon by illuviation, or both. At least
some part of the horizon should show evidence of clay accumulation either as
coatings on surfaces of peds or in pores, as lamellae, or as bridges between
mineral grains.
u User defined
This symbol must be defined with each use. A Bu horizon, for example,
might have an extraordinary accumulation of mangans (manganese oxide coatings).
It has been used in the past as a symbol for “unweathered,” but this appears
redundant for C and R horizon. It was not included in the Internet Glossary of
Soil Science Terms in 2006.
v Plinthite
This symbol indicates the presence of iron-rich, humus-poor, reddish
material that is firm or very firm when moist and hardens irreversibly when
exposed to the atmosphere and to repeated wetting and drying.
w Development of
color or structure
This symbol is used with B to indicate the development of color or
structure, or both, with little or no apparent illuvial accumulation of
material. It should not be used to indicate a transitional horizon.
x Fragipan
character
This symbol indicates a genetically developed layer that has a
combination of firmness and brittleness and commonly a higher bulk density than
the adjacent layers. Some part of the layer is physically root-restrictive.
y Accumulation
of gypsum
This symbol indicates an accumulation of gypsum.
z Accumulation
of salts more soluble than gypsum
This symbol indicates an accumulation of salts that are more soluble than
gypsum.
HUMUS. The well-decomposed, more or less stable part of
the organic matter in mineral soils.
ILLUVIATION. The deposition by percolating water of
solid particles, mostly clay or humus, within a soil horizon.
INTERFLUVE. The land lying between streams.
ISOCHRONOUS BOUNDARY. A gradational boundary between
two sedimentary units indicating that they are approximately the same age.
Opposed to a nonisochronous boundary, which by its abruptness indicates that it
delineates units having significant age differences.
KROTOVINA. An animal burrow filled with soil.
LEACHING. The removal of soluble material from soil or
other material by percolating water.
LOWSTAND. The lowest elevation reached by the ocean
during a glacial period.
MANGAN. A thin coating of manganese oxide (cutan) on
the surface of a sand grain, pebble, soil aggregate, or ped. Mangans also line
pores or root channels and bridge sand grains.
MODERN SOIL. The portion of a soil section that is
under the influence of current pedogenetic conditions. It generally refers to
the uppermost soil regardless of age.
MODERN SOLUM. The combination of the A and B horizons
in the modern soil.
MORPHOLOGY, SOIL. The physical make-up of the soil,
including the texture, structure, porosity, consistence, color, and other
physical, mineral, and biological properties of the various horizons, and the
thickness and arrangement of those horizons in the soil profile.
MOTTLING, SOIL. Irregularly marked with spots of
different colors that vary in number and size. Mottling in soils usually
indicates poor aeration and lack of drainage. Descriptive terms are as follows:
abundance--few, common, and many; size--fine, medium, and coarse; and
contrast--faint, distinct and prominent. The size measurements are these: fine,
less than 5 mm in diameter along the greatest dimension; medium, from 5 to 15
mm, and coarse, more than 15 mm.
MRT (MEAN RESIDENCE TIME.) The average age of the
carbon atoms within a soil horizon. Assuming ideal reducing conditions with
constant humic and fulvic acid production, the humus in a soil will have a C-14
age that is exactly half the true age of the soil. MRT ages, however, must be
used with caution. They may be too young: In oxic soils humus is
typically destroyed as fast as it is produced, generally yielding MRT ages no
older than 300-1000 years, regardless of the true age of the soil. Also note
that the presence of live roots or fauna of any kind indicates that modern
carbon from the atmosphere is being added to the soil regardless of how deep the
horizon is buried. Removal of the roots (both live and dead) will result in an
age closer to the MRT. Unfortunately, the presence of roots indicates that the
buried horizon also must have relatively young humus formed from roots that died
not long ago. Because this young humus cannot be removed from the sample without
removing the old humus we are trying to date, we simply must accept that such
MRT ages still will be too young. Or they may be too old: Charcoal,
either deposited during initial sedimentation or during fires, could be older
than the MRT.
MUNSELL COLOR NOTATION. Scientific description of color
determined by comparing soil to a Munsell Soil Color Chart (Available from
Macbeth Division of Kollmorgen Corp., 2441 N. Calvert St., Baltimore, MD 21218).
For example, dark yellowish brown is denoted as 10YR3/4m in which the 10YR
refers to the hue or proportions of yellow and red, 3 refers to value or
lightness (0 is black and 10 is white), 4 refers to chroma (0 is pure black and
white and 20 is the pure color), and m refers to the moist condition rather than
the dry (d) condition.
OVERBANK DEPOSIT. Fine-grained alluvial sediments
deposited from floodwaters outside of the fluvial channel.
OXIC. A soil having a high redox potential. Such soils
typically are well drained, seldom being waterlogged or lacking in oxygen.
Rubification in such soils tends to increase with age.
PALEO SOIL TONGUE. A soil tongue that formed during a
previous soil-forming interval.
PALEOSEISMOLOGY. The study of prehistoric earthquakes
through the examination of soils, sediments, and rocks.
PALEOSOL. A soil that formed on a landscape in the past
with distinctive morphological features resulting from a soil-forming
environment that no longer exists at the site. The former pedogenic process was
either altered because of external environmental change or interrupted by
burial.
PALINSPASTIC RECONSTRUCTION. Diagrammatic
reconstruction used to obtain a picture of what geologic and/or soil units
looked like before their tectonic deformation.
PARENT MATERIAL. The great variety of unconsolidated
organic and mineral material in which soil forms. Consolidated bedrock is not
yet parent material by this concept.
PED. An individual natural soil aggregate, such as a
granule, a prism, or a block.
PEDOCHRONOLOGY. The study of pedogenesis with regard to
the determination of when soil formation began, how long it occurred, and when
it stopped. Also known as soil dating. Two ages and the calculated duration are
important:
to =
age when soil formation or aggradation began, ka
tb =
age when the soil or stratum was buried, ka
td =
duration of soil development or aggradation, ky
Pedochronological estimates are based on available
information. All ages should be considered subject to +50% variation
unless otherwise indicated.
PEDOCHRONOPALEOSEISMOLOGY. The study of prehistoric
earthquakes by using pedochronology.
PEDOLOGY. The study of the process through which rocks,
sediments, and their constituent minerals are transformed into soils and their
constituent minerals at or near the surface of the earth.
PEDOGENESIS. The process through which rocks,
sediments, and their constituent minerals are transformed into soils and their
constituent minerals at or near the surface of the earth.
PERCOLATION. The downward movement of water through the
soil.
pH VALUE. The negative log of the hydrogen ion
concentration. Measurements in soils are usually performed on 1:1 suspensions
containing one part by weight of soil and one part by weight of distilled water.
A soil with a pH of 7.0 is precisely neutral in reaction because it is neither
acid nor alkaline. An acid or "sour" soil is one that gives an acid reaction; an
alkaline soil is one that gives an alkaline reaction. In words, the degrees of
acidity or alkalinity are expressed as:
Extremely acid------- <4.5
Very strongly acid--- 4.5 to 5.0
Strongly acid-------- 5.1 to 5.5
Medium acid---------- 5.6 to 6.0
Slightly acid-------- 6.1 to 6.5
Neutral-------------- 6.6 to 7.3
Mildly alkaline------ 7.4 to 7.8
Moderately alkaline-- 7.9 to 8.4
Strongly alkaline---- 8.5 to 9.0
Very strongly alkaline >9.0
Used if significant:
Very slightly acid--- 6.6 to 6.9
Very mildly alkaline- 7.1 to 7.3
PHREATIC SURFACE. (See Water Table.)
PLANATION. The process of erosion whereby a portion of
the surface of the Earth is reduced to a fundamentally even, flat, or level
surface by a meandering stream, waves, currents, glaciers, or wind.
PLEISTOCENE. An epoch of geologic time extending from
10 ka to 1.8 Ma; it includes the last Ice Age.
PROFILE, SOIL. A vertical section of the soil through
all its horizons and extending into the parent material.
QUATERNARY. A period of geologic time that includes the
past 1.8 Ma. It consists of two epochs--the Pleistocene and Holocene.
PROGRADATION. The building outward toward the sea of a
shoreline or coastline by nearshore deposition.
RELICT SOIL. A surface soil that was partly formed
under climatic conditions significantly different from the present.
RUBIFICATION. The reddening of soils through the
release and precipitation of iron as an oxide during weathering. Munsell hues
and chromas of well-drained soils generally increase with soil age.
SALINE SOIL. A soil that contains soluble salts in
amounts that impair the growth of crop plants but that does not contain excess
exchangeable sodium.
SAND. Individual rock or mineral fragments in a soil
that range in diameter from 0.05 to 2.0 mm. Most sand grains consist of quartz,
but they may be of any mineral composition. The textural class name of any soil
that contains 85 percent or more sand and not more than 10 percent clay.
SECONDARY FAULT. A minor fault that bifurcates from or
is associated with a primary fault. Movement on a secondary fault never occurs
independently of movement on the primary, seismogenic fault.
SHORELINE ANGLE. The line formed by the intersection of
the wave-cut platform and the sea cliff. It approximates the position of sea
level at the time the platform was formed.
SILT. Individual mineral particles in a soil that range
in diameter from the upper limit of clay (0.002 mm) to the lower limit of very
find sand (0.05 mm.) Soil of the silt textural class is 80 percent or more silt
and less than 12 percent clay.
SLICKENSIDES. Polished and grooved surfaces produced by
one mass sliding past another. In soils, slickensides may form along a fault
plane; at the bases of slip surfaces on steep slopes; on faces of blocks,
prisms, and columns undergoing shrink-swell. In tectonic slickensides the
striations are strictly parallel.
SLIP RATE. The rate at which the geologic materials on
the two sides of a fault move past each other over geologic time. The slip rate
is expressed in mm/yr, and the applicable duration is stated. Faults having slip
rates less than 0.01 mm/yr are generally considered inactive, while faults with
Holocene slip rates greater than 0.1 mm/yr generally display tectonic
geomorphology.
SMECTITE. A fine, platy, aluminosilicate clay mineral
that expands and contracts with the absorption and loss of water. It has a high
cation-exchange capacity and is plastic and sticky when moist.
SOIL. A natural, three-dimensional body at the earth's
surface that is capable of supporting plants and has properties resulting from
the integrated effect of climate and living matter acting on earthy parent
material, as conditioned by relief over periods of time.
SOIL SEISMOLOGIST. Soil scientist who studies the
effects of earthquakes on soils.
SOIL SLICKS. Curvilinear striations that form in
swelling clayey soils, where there is marked change in moisture content. Clayey
slopes buttressed by rigid materials may allow minor amounts of gravitationally
driven plastic flow, forming soil slicks sometimes mistaken for evidence of
tectonism. Soil slicks disappear with depth and the striations are seldom
strictly parallel as they are when movement is major. (See also SLICKENSIDES.)
Soil tectonics.
The study of the interactions between soil formation
and tectonism.
SOIL TONGUE. That portion of a soil horizon extending
into a lower horizon.
SOLUM. Combined A and B horizons. Also called the true
soil. If a soil lacks a B horizon, the A horizon alone is the solum.
STONELINE. A thin, buried, planar layer of stones,
cobbles, or bedrock fragments. Stonelines of geological origin may have been
deposited upon a former land surface.
The fragments are more often pebbles or cobbles than stones. A stoneline
generally overlies material that was subject to weathering, soil formation, and
erosion before deposition of the overlying material. Many stonelines seem to be
buried erosion pavements, originally formed by running water on the land surface
and concurrently covered by surficial sediment.
STRATH TERRACE. A gently sloping terrace surface
bearing little evidence of aggradation.
STRUCTURE, SOIL. The arrangement of primary soil
particles into compound particles or aggregates that are separated from
adjoining aggregates. The principal forms of soil structure are--platy
(laminated), prismatic (vertical axis of aggregates longer than horizontal),
columnar (prisms with rounded tops), blocky (angular or subangular), and
granular. Structureless soils are either single grained (each grain by itself,
as in dune sand) or massive (the particles adhering without any regular
cleavage, as in many hardpans).
SUBSIDIARY FAULT. A branch fault that extends a
substantial distance from the main fault zone.
TECTOTURBATION. Soil disturbance resulting from
tectonic movement.
TEXTURE, SOIL. Particle size classification of a soil,
generally given in terms of the USDA system which uses the term "loam" for a
soil having equal properties of sand, silt, and clay. The basic textural
classes, in order of their increasing proportions of fine particles are sand,
loamy sand, sandy loam, loam, silt loam, silt, sandy clay loam, clay loam, silty
clay loam, sand clay, silty clay, and clay. The sand, loamy sand, and sandy loam
classes may be further divided by specifying "coarse," "fine," or "very fine."
TOPOSEQUENCE. A sequence of kinds of soil in relation
to position on a slope. (See also CATENA.)
TRANSLOCATION. The physical movement of soil particles,
particularly fine clay, from one soil horizon to another under the influence of
gravity.
UNIFIED SOIL CLASSIFICATION SYSTEM. The particle size
classification system used by the U.S. Army Corps of Engineers and the Bureau of
Reclamation. Like the ASTM and AASHO systems, the sand/silt boundary is at 80 um
instead of 50 um used by the USDA. Unlike all other systems, the gravel/sand
boundary is at 4 mm instead of 2 mm and the silt/clay boundary is determined by
using Atterberg limits.
VERTISOL. A soil with at least 30% clay, usually
smectite, that fosters pronounced changes in volume with change in moisture.
Cracks greater than 1 cm wide appear at a depth of 50 cm during the dry season
each year. One of the ten USDA soil orders.
WATER TABLE. The upper limit of the soil or underlying
rock material that is wholly saturated with water. Also called the phreatic
surface.
WAVE-CUT PLATFORM. The relatively smooth, slightly
seaward-dipping surface formed along the coast by the action of waves generally
accompanied by abrasive materials.
WEATHERING. All physical and chemical changes produced
in rocks or other deposits at or near the earth's surface by atmospheric agents.
These changes result in disintegration and decomposition of the material.
WETTING FRONT. The greatest depth affected by moisture
due to precipitation.
yr B.P. Uncorrected radiocarbon age expressed in years
before present, calculated from 1950. Calendar-corrected ages are expressed in
ka, or, if warranted, as A.D. or B.C.
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