Crossword Genius

Archean and Hadean, e.g. (4)

Ross

I believe the answer is:

eons

(Other definitions for eons that I've seen before include "Long periods of time - noes have it" , "Years and years" , "Vast ages" , "Very long periods (US)" , "Divisions of geological time" .)

I've seen this clue in The Wall Street Journal.
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