Scientists use a specialized timeline to understand the history of our planet. This is known as Geologic Time, which is the record of Earth’s history. Because the Earth is billions of years old, we cannot measure its history in days or months. Instead, we look at the layers of rock to see how the world has changed. Every layer of stone acts as a page in a massive history book.

When looking at rocks, we follow a specific rule to tell how old they are. This is called the Law of Superposition, which states that in horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it. This allows scientists to determine the relative age of different events. It is the most basic tool for organizing the history of the ground.

Sometimes, the order of rock layers is changed by volcanic activity. An Extrusion is an igneous rock layer formed when lava flows onto Earth's surface and hardens. Because this lava must flow over existing ground, the extrusion is always younger than the rocks beneath it. Geologists look for these layers to find points where the environment was very hot. These layers help us see when ancient volcanoes were active.

The history of the Earth is not always perfectly preserved in the rocks. An Unconformity is a gap in the geologic record that happens when rock layers are lost due to erosion. Wind and water can wash away millions of years of history before new rock forms on top. This creates a "missing chapter" that scientists have to solve like a puzzle. By comparing different sites, we can eventually fill in these missing pieces.

Inside these rock layers, we often find the remains of ancient creatures. A Fossil is the preserved remains or traces of living things that existed in the past. Most fossils form when an organism dies and is buried by sediment that eventually turns to stone. They provide the only physical evidence we have of what life looked like millions of years ago. Fossils allow us to reconstruct ancient ecosystems and climates.

Some fossils are more helpful than others when we are trying to date a rock layer. An  is a fossil of an organism that was widely distributed and existed for a geologically short period of time. Because these creatures only lived for a brief window, finding one tells a scientist exactly which time period they are looking at. They act like a date stamp on a document. Trilobites are one of the most famous examples of these useful markers.