The bottom layer of a rock in a mountain is the oldest. This is based on the principle of superposition, a fundamental concept in geology.

Here's why:

* Sedimentary rocks form in layers: Rocks like sandstone, limestone, and shale are formed from sediments that settle over time. The oldest layers are deposited first, and newer layers are deposited on top.

* Uplift and erosion: Mountains are formed when layers of rock are uplifted and pushed up. Erosion then carves away the top layers, revealing the older rocks beneath.

So, while the top of a mountain might appear to be the oldest, it's actually the bottom layer that was laid down first and is therefore the oldest.