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While digging the Somerset Coal Canal in southwest England, he found that fossils were always in the same order in the rock layers.
Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating, archaeologists and geologists used relative dating to determine ages of materials.
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 datin g' as observed in geology from the 17th century to the early 20th century.
The regular order of the occurrence of fossils in rock layers was discovered around 1800 by William Smith.
however, this process is not enough to allow the layers to change their positions.
This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.
A similar situation with igneous rocks occurs when xenoliths are found.