Possible Clues:

• River over point burst open
• Explode some powder up there
• Burst, volcano-style
• It needs a pure start so to blow one's top
• Become very angry, being clean upset over end of it
• Time for virgin to return and break out
• Innocent went back to start of tunnel, to break out
• Break through (as a volcano)
• Burst forth, like 25
• How it might come out that this had a chaste turn at first