Related Clues:

• The Phanerozoic, e.g.
• See 41 Down
• Era after era
• It's split into eras
• Quite a long time
• It's longer than an era
• Big time division
• Span of epoch proportions?
• Extensive period
• Immeasurable time
• Geologicial time period
• Billion-year time period
• One billion years, geologically
• Humongous span
• Annoying wait, facetiously