Which law states that the oldest rock layers are usually the deepest?

The principle of superposition or law of superposition states that the deepest rock layers consist of the oldest rocks. The older rocks are overlain by the more recent rocks. This means that if we were to dig into rock strata and start dating the samples, we will find that the age of rock samples increase as we dig deeper. In other words, younger rocks will be found at lesser depths and the older rocks will be found at greater depths. This also means that the fossils found in the deeper layers are older as compared to the fossils found in the upper rock layers.
The law of superposition is useful for understanding the history of our planet, because it enables us to determine the relative ages of the rocks and the fossils found in them.