In stratigraphy, paleontology, geology, and geobiology an erathem is the total stratigraphic unit deposited during a certain corresponding span of time during an era in the geologic timescale. It can therefore be used as a chronostratigraphic unit of time which delineates a large span of years — less than an geological eon, but greater than its ...
Found on
http://en.wikipedia.org/wiki/Erathem
(from the article `era`) ...the Precambrian, or earliest, eras are classified independently. An era is composed of one or more geological periods. The stratigraphic, or rock, ...
Found on
http://www.britannica.com/eb/a-z/e/39
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