Here's why:

* Geological Time Scale: The Earth's history is divided into eons, eras, periods, and epochs. The oldest rocks formed billions of years ago.

* Sedimentation: Sedimentation is the process by which rocks are formed. Over time, layers of sediment build up on top of each other. The oldest rocks are at the bottom, buried beneath newer layers.

* Fossils: Fossils, the preserved remains of ancient organisms, are typically found in sedimentary rocks. The older the rock layer, the older the fossils it contains.

Important Note: This is a general principle. There can be exceptions due to geological processes like faulting, folding, and erosion, which can rearrange and expose layers.