The rock at the bottom of an undisturbed sequence is the oldest. This is a fundamental principle of geology known as the principle of superposition.

Here's why:

* Sedimentation: Sedimentary rocks are formed by the accumulation of sediments over time. As new layers are deposited, they settle on top of older ones.

* Uninterrupted Sequence: In an undisturbed sequence, the layers have remained in their original order since their formation.

* Bottom Layer: The rock at the bottom of this sequence was deposited first, making it the oldest.

Exceptions:

* Faulting: If a fault (a break in the Earth's crust) cuts through the sequence, the principle of superposition may not apply.

* Folding: If the rocks have been folded, the order of the layers may be reversed.

Let me know if you'd like to explore any of these concepts in more detail!