STRATIFICATION

(Lat.,=made in layers), layered structure formed by the deposition of sedimentary rocks. 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.

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