There's One Thing You Never Put In A Trap
The Weeping Angels have the Doctor, River, Amy and the Clerics surrounded and are slowly getting closer. But they've made one fatal mistake, because you see, there's one thing you never put into a trap.
The gravity of the situation
The Weeping Angels have caught up with The Doctor, River and Amy. Their only hope for escape is to send a "complicated space-time event" such as The Doctor himself or the choir of the Angels into the crack. The Angels presence drains the power from the ship Byzantium until the gravity system fails, causing the Weeping Angels to fall into the crack, erasing them from history..