In undisturbed rock formations, the oldest layers of rock are found at the bottom, and the youngest layers are found at the top. This is based on the principle of superposition, a fundamental concept in geology.

Here's why:

* Sedimentation: Rocks are formed over time through the accumulation of sediments. As new sediments are deposited, they bury the older layers beneath them.

* Time: The layers at the bottom were deposited first, making them the oldest. The layers at the top were deposited last, making them the youngest.

Important Note: This principle applies to undisturbed formations. Geological processes like faulting, folding, and erosion can disrupt rock layers and make it difficult to determine their relative ages.