Crossword Genius

One of four in Earth's history (3)

Ross

I believe the answer is:

eon

(Other definitions for eon that I've seen before include "Vast age (alt. sp.)" , "Prolonged period" , "thousand million years" , "Geological period" , "plenty of time" .)

I've seen this clue in The New York Times.
Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.