This is important to stratigraphic dating, which assumes that the law of superposition holds true and that an object cannot be older than the materials of which it is composed.
The law was first proposed in the 17th century by the Danish scientist Nicolas Steno.
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Superposition in archaeology and especially in stratification use during excavation is slightly different as the processes involved in laying down archaeological strata are somewhat different from geological processes.
Now imagine that you come upon a formation like this: What do you think of it? How can you make any conclusions about rock layers that make such a crazy arrangement?
The thick, dark, gray layer at the bottom is made of basalt. You have just used the principle of superposition to interpret the relative ages of the layers.
This principle states that in a sequence of undisturbed sedimentary layers or lava flows, the oldest layers are at the bottom.
Geologists establish the age of rocks in two ways: numerical dating and relative dating.
Numerical dating determines the actual ages of rocks through the study of radioactive decay.