States that for undisturbed rocks, the oldest rocks are on the bottom and the rocks become younger and younger toward the top

The correct answer is - Principle of superposition.

In the geology, the principle of superposition is very often used for the determining of the age of the rocks, but only if the layers are undisturbed. This principle suggests that if the rock layers have not been disturbed, than the layers of rocks are order in a manner from youngest to oldest, top to bottom viewed.

This is a very useful and easy way for determining the age of the rocks, since the new rock layers are coming on top of the older ones, but also, very often in nature that is not the case. The rocks are not always ordered by age, because very often there's disturbance to them, and that can be sideways pressure, pressure from bellow, intrusion, and they all will manage to mix up the layers.