Here's why:

* Fossils as Time Markers: Fossils represent the preserved remains or traces of ancient organisms. Different types of fossils are found in specific rock layers and represent different time periods in Earth's history.

* Correlation: Geologists use the presence and distribution of these fossils to correlate rock layers across vast distances. If similar fossils are found in rock layers in different locations, it suggests those layers formed at roughly the same time.

* Index Fossils: Some fossils are particularly useful for correlation. These are called "index fossils" and are characterized by:

* Wide Distribution: They were found across a large geographic area.

* Short Time Span: They existed for a relatively short period of geological time.

By studying fossils, geologists can piece together the history of life on Earth and understand how different environments have changed over time.