1. Law of Superposition:
* In undisturbed rock sequences, the oldest layers are at the bottom, and the youngest layers are at the top. This is like a stack of pancakes, where the first pancake you put down is at the bottom and the last one is at the top.
2. Fossil Correlation:
* Fossils are often found within specific layers of rock.
* Index fossils are particularly helpful: these are fossils of organisms that lived for a short period of time and were geographically widespread.
* Finding the same index fossil in different rock layers indicates that those layers are roughly the same age.
3. Cross-Cutting Relationships:
* If a geological feature (like a fault or an igneous intrusion) cuts through existing rock layers, it must be younger than the layers it cuts through.
* For example, if a fault cuts through a layer of rock containing a fossil, the fault is younger than the fossil and the rock layer.
4. Unconformities:
* Unconformities represent gaps in the geological record. They form when erosion removes existing rock layers, and then new layers are deposited on top.
* The layers below the unconformity are older than the layers above it.
How it works in practice:
1. Observing rock layers: Scientists examine rock formations and note the order of the layers.
2. Identifying fossils: They look for fossils within the layers.
3. Comparing fossils and layers: They use index fossils and other principles to determine the relative ages of the layers and the fossils they contain.
Important Note: Stratigraphy determines the relative age of fossils. This means it tells us which fossil is older or younger than another, but not the exact age in years. To determine absolute age, scientists use methods like radiometric dating.
