Gaps in the geologic record, called unconformities, are common where deposition stopped and erosion removed the previously deposited material.

Fortunately, distinctive features such as index fossils can aid in matching, or correlating, rocks and formations from several incomplete areas to create a more complete geologic record for relative dating.

Some of the isotopes used for this purpose are uranium-238, uranium-235 and potassium-40, each of which has a half-life of more than a million years.

Unfortunately, these elements don't exist in dinosaur fossils themselves.

If a geologist claims to be 45 years old, that is an absolute age.

Superposition: The most basic concept used in relative dating is the law of superposition.

For example, shells, wood, and other material found in the shoreline deposits of Utah’s prehistoric Lake Bonneville have yielded absolute dates using this method.

These distinct shorelines also make excellent relative dating tools.

Simply stated, each bed in a sequence of sedimentary rocks (or layered volcanic rocks) is younger than the bed below it and older than the bed above it.

This law follows two basic assumptions: (1) the beds were originally deposited near horizontal, and (2) the beds were not overturned after their deposition.

Development of the geologic time scale and dating of formations and rocks relies upon two fundamentally different ways of telling time: relative and absolute.