Related Clues:
- A billion years, geologically
- Very long stretches
- Almost an eternity
- Units of time
- Geological time periods
- Near-infinite timespans
- Lengthy units of time
- Incalculable amounts of time
- Stellar periods
- Long, long periods
- Quite a long time
- How long it takes canyons to form
- Ages and ages and ages
- Immeasurable time
- One billion years, geologically