With the development of stratigraphy, it has been found useful to develop a standard method of nomenclature. The Cambrian System consists of rocks deposited during the Cambrian Period. The system consists of rocks; while the period is the time during which these rocks were formed. Both terms have an age connotation.
Systems are recognized and defined by the fossils they contain. If the fossils within a system can be shown to change regularly with time, systems can be subdivided into series, stages and zones, each of which represent the rocks laid down during shorter intervals of time respectively (as recognized by the fossils).
The systems are grouped together into eras. After the Pre-cambrian, there are three eras: Palaeozoic, Mesozoic and Cenozoic (Table 1). The Cenozoic Era can be divided into the Tertiary (Palaeocene to Pliocene) and the Quaternary.
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