Here's why:
* Fossils are unique and change over time: Different types of organisms lived at different points in Earth's history. This means that fossils found in a rock layer can be used to determine the approximate age of that layer relative to other layers.
* Fossils are widespread: Many fossils are found in geographically widespread locations, allowing for comparisons between rock layers from different areas.
* Index fossils are key: Certain fossils, called index fossils, are particularly useful. These are fossils of organisms that:
* Lived for a relatively short period of time.
* Were geographically widespread.
* Are easily recognizable.
* Are abundant in the rock record.
Other indicators of relative age include:
* Superposition: In undisturbed rock layers, the oldest layers are at the bottom and the youngest layers are at the top.
* Cross-cutting relationships: A rock layer or feature that cuts across another layer is younger than the layer it cuts.
* Unconformities: Gaps in the rock record represent missing time. These gaps can be caused by erosion or non-deposition of sediments.
While all of these indicators are helpful, fossil content provides the most reliable way to determine the relative age of a rock layer.
