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These entries are copied from the summaries of the National Register of Historic Places (NRHP) listing for the site.
For sites which were created in prehistoric periods the period of significance is generally filled out to coorespond to dates of occupation or creation, which may be very specific for a single construction or occupation episode (possibly based on carbon dating) or very general (the date associated with a particular cultural period - an example) based on tool desing or pottery.
In the case of a Rock Art site, the dates may correspond to symbols associated with multiple, successive cultural periods, that have been added to the sites over many eras. Or the dates may be associated with artifacts found in the immediate area that are identified with various cultural periods.
I'd say they add information to the site listing. I will make a minor edit.
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