What is the difference between relative and numerical age dating

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As an example of how they are used, radiometric dates from geologically simple, fossiliferous Cretaceous rocks in western North America are compared to the geological time scale.To get to that point, there is also a historical discussion and description of non-radiometric dating methods.The layers of rock are known as "strata", and the study of their succession is known as "stratigraphy".Fundamental to stratigraphy are a set of simple principles, based on elementary geometry, empirical observation of the way these rocks are deposited today, and gravity.This orientation is not an assumption, because in virtually all situations, it is also possible to determine the original "way up" in the stratigraphic succession from "way up indicators".For example, wave ripples have their pointed crests on the "up" side, and more rounded troughs on the "down" side.This document is partly based on a prior posting composed in reply to Ted Holden.

You can't deform a structure (e.g., bedding) that is not there yet!They are applied by geologists in the same sense that a "null hypothesis" is in statistics -- not necessarily correct, just testable.In the last 200 or more years of their application, they are valid, but geologists do not assume they are.For example, the principle of superposition is based, fundamentally, on gravity.In order for a layer of material to be deposited, something has to be beneath it to support it.

These are often characterised as the norm, rather than the exception.

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