* Sedimentation: Rocks form from sediments that accumulate over time. As new layers of sediment are deposited, they bury the older layers below them. This process is called sedimentation.

* Principle of Superposition: This fundamental principle in geology states that in an undisturbed sequence of sedimentary rocks, the oldest rocks will be at the bottom, and the youngest rocks will be at the top.

Exceptions to this rule:

* Folding and Faulting: If the rock layers have been folded or faulted, the original order of deposition might be disrupted, and the youngest rocks might end up below the oldest.

* Igneous Intrusions: Molten rock (magma) can intrude into existing layers, creating rocks that are younger than the layers they cut through.

* Erosion: Erosion can remove layers of rock, leading to gaps in the geological record and making it difficult to determine the relative age of the remaining layers.

In summary: While the deeper layers are generally older, it's important to consider geological processes that can alter the original order of layers and potentially make the interpretation of rock ages more complex.