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The layers of rock at the base of the canyon were deposited first, and are thus older than the layers of rock exposed at the top (principle of superposition).In the Grand Canyon, the layers of strata are nearly horizontal.A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved.However, by itself a fossil has little meaning unless it is placed within some context.The principle states that any geologic features that cut across strata must have formed after the rocks they cut through (Figures 2 and 3).According to the principle of original horizontality, these strata must have been deposited horizontally and then titled vertically after they were deposited.
Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free.
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.
Second, it is possible to determine the numerical age for fossils or earth materials.
Numerical ages estimate the date of a geological event and can sometimes reveal quite precisely when a fossil species existed in time.