PALEOPEDOLOGY GLOSSARY

Accretionary soil: a thick soil formed by simultaneous slow deposition of sediment and pedogenesis; it may possess an over-thickened A horizon which grades up into overlying unaltered sediment, or an "endless B horizon" formed as the basal A horizon is progressively transformed into B horizon material (synonym: cumulic soil).

Diagenesis: the chemical, physical and biological processes that charge sediments and soils after burial beneath younger deposits. They include compaction, formation of new minerals, redistribution, cementation, reduction and loss of (or changes in) organic matter, but exclude the pedogenetic processes involved in original development of a soil before burial. However, many diagenetic processes are similar to pedogenetic, and the two may be difficult to distinguish. Diagenesis includes all post-burial changes occurring at temperature (up to 200C) and pressures (up to 100 MPa) characteristic of the outer part of the earth's crust; processes at higher temperatures and pressures are metamorphic.

Duricrust: a soil horizon or sequence of consecutive horizons by precipitation of calcium carbonate (calcrete or caliche), magnesium-rich carbonate (dolocrete), calcium sulphate (gypcrete), iron/oxides (ferricrete), aluminium oxides/hydrated oxides (alucrete) or silica (silcrete); it should be at least 1 cm thick and laterally continuous, and may form at any depth in the soil profile.

Geosol: the fundamental unit in pedostratigraphy, consisting of a traceable, mappable three-dimensional body of soil material comprising one or more differentiated pedologic horizons; it has a consistent stratigraphic position, and is defined at a type locality where the horizons are buried be younger deposits, but may be traced to sites where it crops out on the present land surface. Geosols transgress lithostratigraphic or lithodemic units, and change laterally in response to differences in parent material, the original buried topography, drainage, vegetation and other factors, giving different pedifacies, Geosols may be monogenetic, polygenetic or compound (multistory). Compound geosols (sometimes termed pedocomplexes) comprise two or more soils, which are separated over large areas by thin unmodified deposits and are overlain and underlain by greater thicknesses of unmodified deposits or by unconformities; they often merge laterally into polygenetic geosols where the intrageosols deposits are very thin (modified by pedogenesis) or absent.

Lithified soil: soil material which has been hardened by pedogenetic cementation (duricrust) or by diagenetic compaction and/or cementation or by metamorphic processes and cannot be moulded by the fingers or fully dispersed in water.

Monogenetic soil: a soil formed in a period when the variation in environmental factors was too small to produce detectably different assemblages of soil features in different parts of that period (i.e. the direction of soil development was constant).

Paleosol: previously defined (Ruhe, R.V. 1956, Soil Science 82, 441) as "a soil formed on a landscape during the geologic past"; it began to form during the Pleistocene or an earlier geological period and (a) is partly or completely buried deposits that are at least 10,000 radiocarbon years old, or (b) contains distinct evidence that the direction of soil development was different from that of the present. Paleosols are therefore either buried or surface (non-buried or exhumed). Soils entirely formed in and buried during the Holocene period (since 10,000 B.P.) are termed buried soils, not buried paleosols, even if they show evidence of development in a direction different from that of the present. Paleosols and buried soils are often truncated by erosion. Buried paleosols and many buried soils are also subject to diagenetic changes, which may be difficult to distinguish from past or current pedogenetic changes.

Pedoderm: a mappable unit mantle of soil which has physical, chemical or biological characteristics and stratigraphic relationships that permit its consistent recognition and mapping; obsolete (see geosol).

Pedofacies: the different profile types (soil horizon sequences) of a geosol that result from lateral variation in parent material, climate, vegetation, topography or length of development period.

Pedolith: (a) lithified soil, such as laterite or silcrete; obsolete (see lithified soil). (b) transported (allochthonous) soil material; obsolete (see soil sediment).

Polygenetic soil: a soil formed in two or more periods when the environmental factors were sufficiently different to produce detectably different assemblages of soil features (i.e. the directions of soil development were different in the periods involved). Minor episodes of deposition may have occurred between or within the periods of soil development, but were not sufficient to leave layers of unaltered material.

Relict soil: (a) a surface (non-buried) soil containing features formed in an environment different from the present; its development began in a pre-existing landscape and continues today because it was never buried; obsolete (see paleosol, non-buried). (b) soil aggregates transported from their original site of formation; obsolete (see soil sediment).

Soil: a three-dimensional body on the surface of the earth composed of mineral and/or organic material, air and water, and formed by the impact of environmental factors acting on parent materials over a period of time to produce a sequence of horizons. The organic material includes both living and dead components and is at least partly autochthonous. 'Soil' is now obsolete as a stratigraphic term but informally is still widely used as such. It is also used in various slightly different senses by civil engineers, botanists, farmers etc. With slight modification this definition can also be applied to the surface materials of extra-terrestrial bodies such as other planets.

Soil sediment: soil material which has been transported a considerable distance from its original site of formation yet retains some recognizable soil fabric (i.e. was incompletely dispersed during transportation).

Vetusol: a surface soil formed over a very long period (10⁵ - 10⁷ years) during which the land surface remained fairly stable and the variation in environmental factors was too small to produce detectably different assemblages of soil features (i.e. the direction of soil development was constant). These old monogenetic soils occur mainly on peneplains and other old geomorphic surfaces in tropical and subtropical regions where Quaternary climatic changes were small.