ABSTRACT
Geological units are divided in various ways, depending on the characteristics considered and the classification methods. The term stratigraphy describes the ways in which strata are defined. Tables 18.1, 18.2 and 18.3 identify the commonly used stratigraphic terms. Geochronologic units are periods of time, whereas chronostratigraphic units are geological materials. Geochronology is a tool used in chronostratigraphy. Chronostratigraphic units are geological markers or periods of time marked by fossil occurrences that indicate relative ages. Geochronology determines the age of a rock or fossil using markers including fossils, isotopic dating, luminescence dating and palaeomagnetic dating and chemical indicators. The ages determined may be either relative or absolute depending on the methods.
