Geologists use the relative dating rules of cross-cutting and superposition to determine the timing of deposition and deformation. For example, when one sedimentary layer sits above another, it can be inferred that the top layer is most likely younger than the layer below. Furthermore, if a fault or igneous body cuts across a particular rock unit, then the fault or igneous body must be younger than the rock unit it offsets.

Nicolaus Steno, a Danish anatomist, geologist, and priest (1636 - 1686) observed the changes in a sequence of rock layers while working in the mountains of Italy. Steno’s observations became known as the Law of Superposition which stated that in a sequence of sedimentary rock layers, each layer of rock is older than the layer above it and younger than the rock layer below it. The Law of Superposition also applies to other geologic processes on Earth's surface, such as lava flows and ash layers from volcanic eruptions.

The Law of Superposition meant that the Coconino Sandstone is older than the Toroweap Formation and younger than the Hermit Shale as seen in the image above. As it pertained to the Geologic Time Chart, the Law of Superposition meant that the Jurassic Period was older than the Cretaceous Period and younger than the Triassic Period.

https://imnh.iri.isu.edu/exhibits/online/geo_time/geo_principles.htm#super