Dating of sedimentary rocks

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Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization (although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal).The law of superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it.The principle of inclusions and components states that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them.For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer.

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The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut.A similar situation with igneous rocks occurs when xenoliths are found.These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix.however, this process is not enough to allow the layers to change their positions.This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.

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