Geologic time uses the layers of rock to describe the relationships of events and their sequence in chronological order over the history of Earth. The layering is a result of the buildup of tiny bits of rock, sediment, or lava cooling at the surface over many years. Think of it like a stack of pancakes—the bottom layer is the one that was placed first and therefore is the oldest. The sediments that create the layers of Earth are stacked one on top of the other.
to determine the related age of different rocks geologists start with the assumption that unless something has happened in the sequence of sedimentary rock layers the newer Rock layers will be on top of the older ones this is called the rule of superposition with absolute age dating you get a real age and actual years
