Related Clues:

• One billion years, in astronomy
• Long stretches
• Group of eras
• Grows up
• Consent and Reason, e.g.
• Golden and Bronze
• Noteworthy period of history
• Series of periods
• Period between blue moons
• Vast time period
• Big period
• Geologic division
• Longest division of geological time
• Improves Scotch
• Ice and Iron