ALL SEX DATING
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Geologic dating principles
However, as previously noted, times of volcanism and metamorphism, which are both critical parts of global processes, cannot be correlated by fossil content.
Furthermore, useful fossils are either rare or totally absent in rocks from Precambrian time, which constitutes more than 87 percent of Earth history.
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.
For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer.
The most useful indication of time equivalence is similar fossil content, provided of course that such remains are present.
The basis for assuming that like fossils indicate contemporary formation is faunal succession.
The principle 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.
Logically a younger layer cannot slip beneath a layer previously deposited.