Define stratigraphic dating

We know that layer A is older than layer B, even if we have no idea how long ago A and B were deposited.In some cases, things get scrambled and even reversed, but geologists learned to diagnose these aberrant cases early on, as these processes leave obvious clues.

These have recently been supplemented with non-biological markers, such as magnetic polarity reversals or changes in the isotopic ratios of certain key elements (stable isotopes, not radioisotopes -- this technique is only remotely related to radiometric dating). Isotopes of uranium and thorium are almost the only materials useful for dating sediments of geologically interesting age. Why do we bother with all these weird names for different time slices?Actually, almost all of the time periods in the Precambrian are based on radiometric dates. As soon as stratigraphers can find enough information, they will change the simple date ranges to more complex entities defined in some other way.Instead, stratigraphers normally use small animals or (more often these days) microfossils.Virtually no sediment created during the last 600 My is so sterile that it completely lacks the remains of these organisms: conodonts, graptolites, tiny mollusk or brachiopod shells, ostracodes, radiolarians, echinoid stem fragments, acritarchs, the rasping teeth of marine worms -- the list goes on and on.

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