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How Fossils are Dated is an excerpt from Introduction to Fossil Collecting by Glen Kuban.

The age of a fossil may be specified in both relative terms (how old it is in relation to other fossils or rock units) and in absolute terms (approximately how many years old it is). One principle of relative dating is called superposition, which holds that in any one place, the lower rock layers (and fossils in them) are older than higher ones, unless there is evidence that the layers have been overturned. This is logical, since the lower sediments would have been deposited first. A related concept is correlation, whereby rock layers (strata) are compared with others in another location on the basis of their mineral composition, fossil content, and other features. When a rock unit or series closely matches those of another area, they are said to be correlated and considered to be of similar age. Correlations are especially reliable when they include index fossils, which are fossils with a limited age range. Good index fossils are easily recognized and widely distributed around the world, so that they aid both local and international correlations.

Absolute dating complements relative dating by providing a specific (not necessarily precise) chronological age for a given specimen, such as "50 million years before present." In recent years reliable forms of absolute dating became available through the development of radiometric dating methods. These methods are based on the known, regular decay of certain radioactive elements (isotopes) into other isotopes or "daughter products." By measuring the amount of "parent" and "daughter" products in a rock sample, its approximate age may be calculated.

Many people have heard of the Carbon 14 method and assume that fossils are dated by it. Actually, in most cases the C-14 method is useful only on organic material less than about 50,000 years old, which includes many human remains and artifacts, but excludes most fossils. Absolute dating of fossils requires other dating methods such as the potassium-argon or rubidium-strontium methods, which involve isotopes with slower decay rates (longer "half-lives"). Such isotopes are rare in fossils themselves, but may occur in surrounding or adjoining rock layers, yielding an approximate age for the fossil-bearing unit. Using the principles discussed above, scientists also can deduce the approximate age of other layers correlated with the same formation, as well as an approximate age for layers above and below the formation (which would be somewhat younger and older respectively).

The terms "above" and "below" in this context are used in a relative or stratigraphic sense; that is, they are based on the positions of the units relative to each other (or correlated units), rather than their height above sea level. The branch of geology that deals with the correlation of rock layers is known as stratigraphy (or biostratigraphy when focusing on the fossils within the layers), and is an important tool of paleontologists.