Related Clues:

• Long interval
• 1,000,000,000 years
• One billion years, in geologic time
• Waiting period, it sometimes seems
• 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
• "Architecture aims at __": Wren
• Era after era