Rock Layers

Sedimentary rocks are one of the three basic rock types. Metamorphic and igneous rocks are the other two. Sedimentary rocks are significant in that they commonly contain fossils.

Most sedimentary rocks are deposited underwater as layers, also called beds or strata. The study of rock layers is called Stratigraphy.

In order to use rock layers to learn about life in the past, geologists and paleontologists rely on a few principles:

1. Superposition: This principle states that in a "stack" of sedimentary layers, the oldest layer is at the bottom of the stack and the youngest layer is at the top. We can use the principle of superposition to compare the relative ages of different rock layers and the fossils found within them.
2. Uniformitarianism: Another important concept we can use when looking at sedimentary rocks is the principle of uniformitarianism. This principle states that the laws that operate today are the same ones that operated in the past. Simply put, "the present is the key to the past." We can use what we know about processes we see today to interpret geologic features from the past.

Here's an example of a uniformitarian interpretation: A geologist finds a mound-like rock structure with abundant fossil corals. They wonder how this structure formed. They know modern corals live only in seawater. Using this knowledge, the geologist infers that this rock structure also formed in seawater. The geologist makes more comparisons of the structure with modern marine environments and discovers its similarity to coral-built reefs of tropical areas.

Stratigraphy is the branch of geology concerned with the order and relative position of strata and their relationship to the geological time scale.