What is relative dating of fossils
Using these key or index fossils as markers, geologists began to identify a particular layer of rock wherever it was exposed.
Therefore, the actual length of geologic time represented by any given layer is usually unknown or, at best, a matter of opinion.
William Smith's collecting and cataloging fossil shells from rocks led to the discovery that certain layers contained fossils unlike those in other layers (see: fossil sorting).
Between the years of 17, James Hutton and William Smith advanced the concept of relative dating.
Hutton, a Scottish geologist, first proposed formally the fundamental principle used to classify rocks according to their relative ages.
He concluded, after studying rocks at many outcrops, that each layer represented a specific interval of geologic time.
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Further, he proposed that wherever un-contorted layers were exposed, the bottom layer was deposited first and was, therefore, the oldest layer exposed; each succeeding layer, up to the topmost one, was progressively younger.