Bedding

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The term bedding (also called stratification) ordinarily describes the layering that occurs in sedimentary rocks and sometimes the layering found in metamorphic rock. Bedding may occur when one distinctly different layer of sediment is deposited on an older layer, such as sand and pebbles deposited on silt or when a layer of exposed sedimentary rock has a new layer of sediments deposited on it. Such depositions of sediments produce a clear division between beds called the bedding plane.

The variation among different sedimentary rock layers (usually referred to as beds or strata) may range from subtle to very distinct depending upon color, composition, cementation, texture, or other factors. One of the best examples may be seen in Arizona's Grand Canyon where red, green, white, gray, and other colors heighten the contrast between beds.

The bedding found in metamorphic rock that formed from sedimentary rock is evidence of extreme heat and pressure and is often quite distorted. Distortions may change the sedimentary bedding by compressing, inclining, folding, or other changes.

One of the most common types of bedding is called graded bedding. These beds display a gradual grading from the bottom to the top of the bed with the coarsest sediments at the bottom and the finest at the top. Graded bedding often occurs when a swiftly moving river gradually slows, dropping its heaviest and largest sediments first and lightest last. Changes in a river's speed may be caused by a number of factors, including storm runoff or the entry of a river into a lake or an ocean.

Bedding is usually found in horizontal layers called parallel bedding. But bedding may be inclined or have a swirly appearance. Inclined bedding may occur when sediments are deposited on a slope, such as a sand dune, or when beds are tilted from their original horizontality by forces within the earth. Bedding with a swirly appearance, called cross bedding, may indicate that the sediments making up the rock were deposited by strong desert winds or turbulence in a river.

The origin, composition, and interpretation of variations in bedding are one of the geologist's most important tools in studying Earth's history. It is for this reason that stratigraphy, the study which includes the interpretation of sedimentary and metamorphic beds, was an essential part of even the earliest days of geologic research.

Superposition