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stratum
geology
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Also known as: strata
rock strata
stratum, sedimentary rock layer bounded by two stratification planes, the latter being produced by visible changes in the grain size, texture, or other diagnostic features of the rocks above and below the plane. A stratum that is less than one centimetre (0.4 inch) in thickness is termed a lamina, whereas one greater than this thickness is a bed. See stratification.