Geologists have established a set of principles that can be applied to sedimentary and volcanic rocks that are exposed at the Earth's surface to determine the relative ages of geological events preserved in the rock record.For example, in the rocks exposed in the walls of the Grand Canyon (Figure 1) there are many horizontal layers, which are called strata.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.As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils.The study of strata is called stratigraphy, and using a few basic principles, it is possible to work out the relative ages of rocks.
In the Grand Canyon, the layers of strata are nearly horizontal.
In addition to being tilted horizontally, the layers have been faulted (dashed lines on figure).
Applying the principle of cross-cutting relationships, this fault that offsets the layers of rock must have occurred after the strata were deposited.
" First, the relative age of a fossil can be determined.
Relative dating puts geologic events in chronological order without requiring that a specific numerical age be assigned to each event.
Sometimes sedimentary rocks are disturbed by events, such as fault movements, that cut across layers after the rocks were deposited.