Related Clues:

• Longest division of time, in geology
• Mellows, maybe
• Large timeline segment
• Improves in the cask
• Astronomical stretch
• Gets older iron, and space-ice perhaps?
• Ice and Stone, e.g
• Long, long, long time period
• 'Rock of --'
• Vast span
• Census information
• Giga-year
• A billion years, in astronomy
• Reeeeeally long time
• An extremely long period of time