Geologic time scale relative dating
(The other types of rock are igneous and metamorphic.) Aside from stratigraphy, discussed in a separate essay, other relative dating techniques include seriation, faunal dating, and pollen dating, or palynology.Used, for instance, in archaeological studies, seriation analyzes the abundance of a particular item (for instance, pieces of pottery) and assigns relative dates based on this abundance.Several factors influence the rate of conversion, and though amino-acid racimization was popular in the 1970s, these uncertainties have led scientists to treat it with increasing disfavor.The principles that undergird amino-acid racimization, however, are essential to most forms of absolute dating.The scale of time for various substances, however, differs greatly.
To discuss the divisions of geologic time, it is necessary first to discuss the concepts of relative and absolute time.
One of the principal means of relative dating is through stratigraphy, which is based on the assumption that the deeper a layer of rock lies beneath Earth's surface, the earlier it was deposited.
This holds true, however, for only one of the three major types of rock: sedimentary rock, which is formed by compression and deposition (i.e., formation of deposits) on the part of rock and mineral particles.
Amino acids exist in two forms, designated L -forms and D -forms, which are stereoisomers, or mirror images of each other.
Virtually all living organisms (except some microbes) incorporate only the L-forms, but once the organism dies the L-amino acids gradually convert to D-amino acids.