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List E - Geologic age (stratigraphic) terms
The notes under Searching attempt to guide the searcher in the use of the list. Searchers might also read the notes on Indexing for further clues but should be aware that these notes reflect current practice which in some cases differs from past practice. Further notes on specific terms are in the body of the Thesaurus and additional notes on searching are in the GeoRef Thesaurus, 11th edition Introduction section on searching beginning on page x. Information specific to searching and the individual list topics is included, e.g., a section including geologic age terms begins on page xi.
Under Indexing the current indexing practice is given. These notes should be read along with the instructions under the individual terms in the body of the Thesaurus.
Geologic age (stratigraphic) terms
Throughout, the "stratigraphic" upper, middle, and lower rather than the "chronologic" early, middle, and late are used to modify age terms, e.g. Upper Cambrian instead of Late Cambrian. "Uppermost" and "lowermost" are not used. Many of the stratigraphic terms are listed in the alphabetical list. The alphabetical list does not include some ancient lakes and ice sheets. Rock unit terms are found in the main body of the Thesaurus and in List R.
Most of the terms include complete autoposting to the level of the Eon for the Precambrian, e.g. Proterozoic; and the Period for the Paleozoic, Mesozoic, and Cenozoic, e.g. Ordovician, Triassic, Paleogene. In a few cases, the hierarchies are completed by supplemental indexing.
A general outline of the age terms used is in the hierarchical list.
In searching for certain broad terms such as Cretaceous, it suffices to use the term as is and not worry about subdivisions, since these automatically post Cretaceous. This applies to all the Eras and Periods and to the Precambrian Eon.
"Upper", "middle", and "lower" are capitalized for Mesozoic and Paleozoic periods (e.g. Upper Cretaceous). This practice does not apply to Precambrian and Cenozoic subdivisions. See the hierarchical list.
Where an age in years is given, use the chart of Major geochronologic and chronostratigraphic units to assign a geologic age. You can use the Geological Time Table to find lunar ages.
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