Related Clues:

• Historical division
• Mighty long time
• Quite a spell
• One billion years, in astronomy
• Group of eras
• Noteworthy period of history
• Series of periods
• Vast time period
• Big period
• Geologic division
• Longest division of geological time
• Forever, figuratively
• Noteworthy period
• Time measured in ages
• Geologist's division