What can the layers of the earth teach us about its history?

The layers of the rocks are the pages in our history book. Most of the rocks exposed at the surface of Earth are sedimentary formed from particles of older rocks that have been broken apart by water or wind. The gravel, sand, and mud settle to the bottom in rivers, lakes, and oceans.

It's as if rock layers are a vertical timeline. At the bottom of the timeline there are no fossils of modern animals. As you move towards the surface, you find fish, then amphibians, then reptiles, etc. Finally modern mammals including humans

In 1669, Nicolaus Steno, a Danish Geologist, described two basic geologic categories: sedimentary rocks are laid down in a horizontal way, with newer rocks laying on top of older rocks. The oldest layer is laid down first, with the newest layer at the top. Scientists call the study of rock layering "stratigraphy."

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