My first dog Callie did the same sort of thing when she was about six months old, fell down the side of a old quarry,which is known locally as 'half mile hill' it was about a 60 ft drop.
We'd taken her out for a run and went up the track that runs along the top of the quarry, one side was a sheer drop, Callie went over to the side,I told her to come back but she went too far and slipped over the edge, even now I can still remember seeing the sheer panic on her face.
She roly-polied from the top to the bottom, my bf at the time ran full pelt back down the hill, and we were sure she was dead, but she just got up and ran about whining and wetting herself, bless her.
Although she was shaking, she only had a slight limp so we took her straight to the vets to get her checked and he diagnosed a slight sprain in her leg.
The Vet said she was very lucky and it was probably the fact she was a puppy that saved her as her ligaments, bones etc were softer because she was still growing.
I have never let any of my dogs off the lead there again.