Answer: It is the way you can date layers of rocks, sediment and fossils. The most recent layer is the youngest in geologic time, while the bottom layer is always the oldest.
Step-by-step explanation:
a basic law of geochronology, stating that when there is any undisturbed sequence of rocks deposited in layers, the youngest layer is on top and the oldest on bottom, each layer being younger than the one beneath it and older than the one above it.