The law of superposition is one of the principles of geology scientists use to determine the relative ages of rock strata, or layers. This principle states that layers of rock are superimposed, or laid down one on top of another. The oldest rock strata will be on the bottom and the youngest at the top.

Think about it like this: You have a magazine subscription and get one magazine in the mail every month. You read the January magazine and place it in a box when you are finished. When you complete the February magazine you place it on top of the January issue. This continues, month after month, until December. Now when you look in the box, you will see only the December issue, which lies on top of the stack of magazines. Below are eleven others, in order, with the oldest issue—January—on the bottom and the youngest magazine—December—on top. So it is with rock strata.

This seems like a simple and even obvious concept, but it was a new idea in 1669. That is when a Danish naturalist named Nichlaus Steno published his theory that older layers of sedimentary rocks are buried deeper in the planet than younger ones, which were laid down horizontally above the older strata. This now fundamental principle became known as the Law of Superposition, and it is considered a basic law of geology.

Using this principle, geologists were able to determine which rock layers were the oldest long before the technology existed to calculate the absolute ages of rocks. It is, however, important to know that before applying the Law of Superposition, a geologist must determine how the layers formed. If the layers of rock are the result of sedimentation then you can assume that the layers were deposited bottom to top. However, if the layers of rock are, for example, metamorphic, then the relative age of the layers can be very different because those layers developed not from deposition, but from the application of pressure. That means all those metamorphic rock layers could have formed at the same time, so the top layer may not be younger than the bottom layer. It is also important to know that sedimentary rock layers may also be out of sequence if the layers have been changed—folding, intrusions, and crosscutting are just some methods that can alter the layers of rock.

Paleontologists also use Steno’s principle of superposition. They dig through layers of sedimentary rocks to excavate fossils and are able to organize them chronologically by noting the strata in which they are found. It also works the other way around: Fossils can be a key to determining the relative ages of sedimentary rocks. The order in which fossils associated with particular time periods appear and disappear in rock layers provides information about the ages of those strata.