Relative dating principles geology
Relative dating does not provide actual numerical dates for the rocks.
Fossils are important for working out the relative ages of sedimentary rocks.
The Proterozoic rocks of the Grand Canyon Group have been tilted and then eroded to a flat surface prior to deposition of the younger Paleozoic rocks.
The difference in time between the youngest of the Proterozoic rocks and the oldest of the Paleozoic rocks is close to 300 million years.
Some of the most useful fossils for dating purposes are very small ones.
For example, microscopic dinoflagellates have been studied and dated in great detail around the world.
Many of these organisms have left their remains as fossils in sedimentary rocks.For example, the principle of superposition states that sedimentary layers are deposited in sequence, and, unless the entire sequence has been turned over by tectonic processes or disrupted by faulting, the layers at the bottom are older than those at the top.The principle of inclusions states that any rock fragments that are included in rock must be older than the rock in which they are included.[SE] The principle of cross-cutting relationships states that any geological feature that cuts across, or disrupts another feature must be younger than the feature that is disrupted.An example of this is given in Figure 8.7, which shows three different sedimentary layers.