Related Clues:

• '___ Flux'
• Unimaginably long time (Var.)
• Proterozoic, for one
• Vast time unit
• D_ting concerns
• Unimaginably long stretches
• Improves, as wine
• Turns yellow or mellow
• Largest division of geologic time
• One billion years (Var.)
• Earth's age, perhaps
• Geologic procession
• Long stretch of history
• Centuries on end
• Becomes less green