Two scientists disagree about a layered rock outcrop containing fossils.

Geologist A: The layers were deposited slowly, one on top of another over millions of years, so deeper layers are always older than shallower ones (the principle of superposition holds undisturbed here).

Geologist B: This outcrop was overturned by tectonic folding, so the current top-to-bottom order is reversed relative to the original deposition order.

A fossil species known from independent dating to be older is found in the TOP layer, while a species known to be younger is found in the BOTTOM layer.