Isotope dating techniques
The principle states that any geologic features that cut across strata must have formed after the rocks they cut through (Figures 2 and 3).
As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils.
This is the principle of original horizontality: layers of strata are deposited horizontally or nearly horizontally (Figure 2).
Thus, any deformations of strata (Figures 2 and 3) must have occurred after the rock was deposited.
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.