Here's how it works:

* Principle of Superposition: In a sequence of undisturbed sedimentary rocks, the oldest rocks are at the bottom and the youngest rocks are at the top. This is based on the idea that sediment is deposited in layers over time.

* Underlying: This refers to the layers of rock that are below other layers.

* Overlying: This refers to the layers of rock that are above other layers.

Superposition is a fundamental principle in geology and helps us understand the relative ages of rocks and fossils. It's often used in conjunction with other dating methods like radiometric dating to create a more complete timeline of Earth's history.