Here's why:

* Sedimentation: Sedimentary rocks, like those forming the Grand Canyon walls, are created by layers of sediment (sand, mud, etc.) being deposited over time.

* Time: The bottom layers were deposited first, and therefore are older than the layers above them.

* Fossils: Fossils are often preserved within these layers of sediment. So, a fossil found in a lower layer is likely older than one found in a higher layer.

Important Notes:

* Exceptions: There can be exceptions to this rule, like faults (breaks in the rock layers) or tectonic activity that can shift the layers.

* Dating: The age of fossils is often confirmed using radioactive dating methods.

Therefore, if a leaf fossil is found below a shell fossil in the Grand Canyon walls, it's likely that the leaf fossil is older.