Related Clues:

• Waiting period, it sometimes seems
• Passes the time?
• Division of geological time
• Long geological period
• An age
• It keeps going and going ...
• Many many years
• A million millennia
• Untold millennia
• The Phanerozoic, e.g.
• See 41 Down
• Era after era
• It's split into eras
• With 37-Down, way back when
• Quite a long time