Relative geologic dating and numerical geologic dating

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.

Relative geologic age is established, based on such evidence as the order in which layers of sediment are stacked, with the younger layer originally on top.

By using the principles of relative geologic age, the sequence of geologic events -- what happened first, what happened next, what happened last -- can be established.

The numerically calibrated geologic time scale has been continuously refined since approximately the 1930s (e.g., Holmes, 1937), although the amount of change with each revision has become smaller over the decades (see fig.

1.5 and 1.6 of Harland ) and a few numerical estimates were available previously (but often for the duration of the entire scale rather than its individual subdivisions).

The overall duration and relative length of these large geologic intervals is unlikely to change much, but the precise numbers may "wiggle" a bit as a result of new data.

The example used here contrasts sharply with the way conventional scientific dating methods are characterized by some critics (for example, refer to discussion in "Common Creationist Criticisms of Mainstream Dating Methods" in the Age of the Earth FAQ and Isochron Dating FAQ).

A common form of criticism is to cite geologically complicated situations where the application of radiometric dating is very challenging.

Initially, the students are asked to develop physical clues to put themselves in order from youngest to oldest (exposing the inferences we make unconsciously about people's ages), and this will be refined/modified using a list of current events from an appropriate historical period that more and more of the students will remember, depending on their age (among other variables).

Absolute age is introduced by having the students order themselves by birth decade, year, month, and day, and comparing the absolute age order to the order worked out in the relative-dating exercise, with a discussion of dating precision and accuracy.

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