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.
Furthermore, useful fossils are either rare or totally absent in rocks from Precambrian time, which constitutes more than 87 percent of Earth history.
Precambrian rocks must therefore be correlated by means of precise isotopic dating.
The basis for assuming that like fossils indicate contemporary formation is faunal succession.
However, as previously noted, times of volcanism and metamorphism, which are both critical parts of global processes, cannot be correlated by fossil content.
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.
Relative dating is the science determining the relative order of past events, without necessarily determining their absolute age.
In geology rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another.
Although with clever detective work many complex time sequences or relative ages can be deduced, the ability to show that objects at two separated sites were formed at the same time requires additional information.
A coin, vessel, or other common artifact could link two archaeological sites, but the possibility of recycling would have to be considered.
From top to bottom: Rounded tan domes of the Navajo Sandstone, layered red Kayenta Formation, cliff-forming, vertically jointed, red Wingate Sandstone, slope-forming, purplish Chinle Formation, layered, lighter-red Moenkopi Formation, and white, layered Cutler Formation sandstone.