The Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of 'relative dating' as observed in geology from the 17th century to the early 20th century.The regular order of occurrence of fossils in rock layers was discovered around 1800 by William Smith.For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer.A similar situation with igneous rocks occurs when xenoliths are found.
Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization (although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal).
Though relative dating can only determine the sequential order in which a series of events occurred, not when they occur, it remains a useful technique especially in radiometric dating.
Relative dating by biostratigraphy is the preferred method in paleontology, and is in some respects more accurate (Stanley, 167–69).
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