Related Clues:
- Astronomer's time period
- Long time period
- Long interval
- 1,000,000,000 years
- One billion years, in geologic time
- Waiting period, it sometimes seems
- Division of geological time
- Unit of geologic time
- Long geological period
- An age
- Part of a period, geologically
- It keeps going and going ...
- Many many years
- A million millennia
- Untold millennia