Related Clues:

• Untold eras
• Inconceivable stretch
• Pleistocene, e.g.
• Period on a geologic time scale
• Distinctive historical period
• Very long interval
• Long while
• Extremely long stretch
• Indefinitely long period
• Geologic span
• A billion years, in geology
• A billion years, to geologists
• Many generations
• Umpteen millennia
• Mind-boggling time span