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stratification(Lat.,=made in layers), layered structure formed by the deposition of sedimentary rocksrock,
aggregation of solid matter composed of one or more of the minerals forming the earth's crust. The scientific study of rocks is called petrology. Rocks are commonly divided, according to their origin, into three major classes—igneous, sedimentary, and metamorphic.
..... Click the link for more information. . Changes between strata are interpreted as the result of fluctuations in the intensity and persistence of the depositional agent, e.g., currents, wind, or waves, or in changes in the source of the sediment. Changes in the mineral composition between two adjacent layers will often result in two layers of distinctly different color. Changes in the texture of the sedimentary particles from one layer to another (as from sand to gravel) result in the development of prominent stratification. In shales, stratification can be seen by the tendency of the rock to split into thin flakes, caused by the parallel arrangement of the tiny clay mineral fragments. Initially, most sediments are deposited with essentially horizontal stratification, although the layers may later be tilted or folded by internal earth forces. Persistent, regular stratification is a reflection of the persistence and regularity of the depositional agent. Agents such as broad ocean or atmospheric currents tend to produce widespread and uniform strata, whereas currents that operate over limited areas and show evidence of turbulence, such as stream currents or irregular wind patterns, form irregular strata.
stratificationsee SOCIAL STRATIFICATION.
the disposal of sedimentary rocks in the earth’s crust in strata or layers. Initial stratification is ordinarily almost horizontal. As a result of tectonic movements of the earth’s crust, stratification may become inclined, folded, or broken by faults.
in agriculture, a presowing method of accelerating the germination of seeds. Stratification is used mainly for difficult-to-sprout seeds of woody species (fruit, forest, and ornamental species) and certain medicinal plants. The seeds are stored in layers in a moist well-aerated medium (sand, sawdust, crumbled peat, moss) and kept at low temperatures (I° to 5°C). Three to four parts substrate are used to one part seeds. The seeds are stratified for one to several months.
(also layering), the vertical division of plant communities, or phytocoenoses, into strata, that is, into rather precisely delimited levels of concentration of the active organs of the plants.
Aboveground stratification is the result of the selection of species capable of growing compatibly by using different levels of the aboveground environment. For example, some plants prosper under conditions of decreased light intensity caused by shading from taller plants. Thus, stratification ensures more complete use of the aboveground environment by components of phytocoenoses.
Stratification is most marked in forests of the temperate zone, where the following strata are distinguished: arboreal (sometimes two arboreal strata), shrub, herbaceous (or herbaceous-shrub), and moss (or lichen). Strata may be formed by one or two or more species, including those belonging to different ecobiomorphs, for example, the arboreal stratum of coniferous and deciduous trees in mixed forests. A stratum is defined when the composition of plants within it is quite closed. Each stratum is characterized by special conditions of the environment, for example, specific light and temperature conditions. Epiphytes and lianas do not belong to any stratum. Young and stunted individuals of tall plant species are usually assigned to the stratum in which their active organs are located.
In some types of phytocoenoses (most herbaceous ones, certain types of tropical rain forests and broad-leaved forests) stratification is absent or weakly expressed; absent or weak stratification has strong implications for the structure of phytocoenoses. Strata are formed early in the establishment of phytocoenoses. They subsequently represent a relatively stable formation (for example, the spruce stratum in spruce forests), or they are subject to changes in the course of the vegetative season (for example, strata formed of herbs or deciduous trees) or sometimes even from year to year (herbaceous polydominant strata).
Depending on the depth of rooting, subterranean stratification is sometimes discerned in phytocoenoses. One stratum is readily distinguishable. Only rarely can a second stratum be discerned, for example, on solonetzes, in arid regions, or in places where groundwaters are close to the surface. The second stratum is an aggregation of absorptive roots in a soil moistened by groundwater.
T. A. RABOTNOV