Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free.These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth's surface is moving and changing.

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For example if you have a fossil trilobite and it was found in the Wheeler Formation.

The Wheeler Formation has been previously dated to approximately 507 million year old, so we know the trilobite is also about 507 million years old.

Relative dating by biostratigraphy is the preferred method in paleontology and is, in some respects, more accurate.

The Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of 'relative dating' as observed in geology from the 17th century to the early 20th century.

As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils.

A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved.

This 5-12-grade activity introduces students to the idea of sequencing information in overlapping data sets and the Principle of Superposition, which is a core concept in relative dating. Offers history of age dating, stratigraphic principles, rock correlation, fossil correlations, radiometric dating, and the geologic time scale. Short discussion of radioactive dating and stratigraphic principles.

Radiometric Dating and the Geologic Time Scale, The Talk Origins Archive. Provides brief overview of (1) relative dating and stratigraphic methods, (2) absolute dating and radiometric dating, including a table with parent to daughter isotopes and half lives of those isotopes commonly used in radiometric dating, (3) paleomagnetics and (4) geologic time. Includes tables of common radioactive parent isotopes and their stable daughter products, and half lives of common radioactive isotopes.

Index fossils are fossils that are known to only occur within a very specific age range.

The Permian through Jurassic stratigraphy of the Colorado Plateau area of southeastern Utah is a great example of Original Horizontality and the Law of Superposition, two important ideas used in relative dating.

The site also provides fact sheets on the age of the Earth and isochron dating.