Related Clues:
- Some collars and jackets
- Billion-year increments, in geology
- 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
- Reeeeeeally long time
- Longest division of geologicalà time
- Gigayear