Long before geologists tried to quantify the age of the Earth they developed techniques to determine which geologic events preceded another, what are termed "relative age” relationships.These techniques were first articulated by Nicolas Steno, a Dane living in the Medici court of Italy in the 17th C.The table below summarises key features: Gastrioceras listeri is a particularly good example of a ZONE fossil.As it is free swimming it could have travelled a considerable distance.Geologic time covers the whole sweep of earth's history, from how and when the earth first formed, to everything that has happened on, in, and to the planet since then, right up to now.Geologists analyze geologic time in two different ways: in terms of relative geologic age, and in terms of absolute (or numeric) geologic age.These ages have been derived from relative dating and absolute dating (radiometric dating) of rock layers and fossils.(a) Relative Dating This technique uses principles of stratigraphy (rock strata) and the study of fossils (palaeontology) to determine the relative ages of rocks and sediments. Field geologists' rely on a number of simple techniques for dating rocks and constructing geological successions. The Law of Strata Identified by Fossils is a little bit more complex.
The textbooks speak of the radiometric dating techniques, and the dates themselves, as factual information.The most obvious feature of sedimentary rock is its layering.This feature is produced by changes in deposition over time.With out individual time stamps the process of dating these structures could become extremely difficult.To deal with many of these problems geologists utilize two types of geologic time: relative time and absolute time.