Mr Marshall said there were no skid marks from the car, so he believed the man aquaplaned.
"There was torrential rain and he was not used to the road nor the car he was driving, as he had borrowed it," Mr Marshall said.
Mr Marshall said the male driver believed he had fallen unconscious after they crashed, as he woke to his mother screaming ''get out of the car''.
Luckily the driver was able to scramble up the vertical bank in bare feet and wave down some cars which pulled over and notified emergency services.
Mr Marshall said police were surprised the man made it up the overhanging cliff, because the bank was so steep. He described it as a vertical drop.
"If he hadn't made it up there it would have been a lot worse. The car was nestled in to the bank by the stream and there were no marks on the road showing traces of the crash, so no one would have seen it."
He said a person must walk to the shoulder of the road to see the car.
The level three abseiling rescue team from the fire service abseiled down and brought the woman back up on a stretcher. She had somehow managed to clamber to the top of the car.