organisms, geologists employ a variety of techniques.
These include some that establish a relative chronology in which occurrences can be placed in the correct sequence relative to one another or to some known succession of events.
Correlating two separated outcrops means establishing that they share certain characteristics indicative of contemporary formation.
The most useful indication of time equivalence is similar fossil content, provided of course that such remains are present.
For example, the presence of recycled bricks at an archaeological site indicates the sequence in which the structures were built.
Similarly, in geology, if distinctive granitic pebbles can be found in the sediment beside a similar granitic body, it can be inferred that the granite, after cooling, had been uplifted and eroded and therefore was not injected into the adjacent rock sequence.
So, instead of saying when something happened, it puts events in the order they happened.
When information derived from two outcrops is integrated, the time interval they represent is probably greater than that of each alone.
Prior to the discovery of radiometric dating which provided a means of absolute dating in the early 20th century, archaeologists and geologists were largely limited to the use of relative dating techniques to determine the geological events.
Though relative dating can only determine the sequential order in which a series of events occurred, not when they occur, it remains a useful technique especially in materials lacking radioactive isotopes.
Because the earth is so old and rocks formed millions of years ago, geologists needed a way to date rocks and rock units called strata.
Today there are two common practices for dating rocks and strata. Geologists use what they see and some simple strategies to relative date the rock layers found in the Grand Canyon.