Why would fossils at the top of a canyon be younger than at the bottom?

Fossils at the top of a canyon are younger than those at the bottom due to the principle of stratigraphy, which states that in undisturbed sedimentary layers, the oldest layers are at the bottom and the youngest are at the top. As sediment accumulates over time, new layers are deposited on top of older ones. Therefore, the fossils found in the higher layers were formed more recently than those in the lower layers, reflecting the chronological sequence of geological events.