We follow this same idea, with a few variations, when we talk about cross-cutting relationships in rock. Sedimentary rocks frequently contain objects that have been interpreted as evidence that life existed at the time the sediment accumulated. If certain fossils are typically found only in a particular rock unit and are found in many places worldwide, they may be useful as index or guide fossils in determining the age of undated strata. That is not surprising since more than one type of organism lives at the same time. The newer layers would have formed a cap overtop.
A second observer, who has not been to location A, sees slightly inclined layers and concludes correctly that the layers have been somewhat deformed, but that the topmost layer is the youngest and the bottommost the oldest.
Geologic Age Dating Explained