According to local legend, Mr. Proctor was a Crummackdale farmer who tended to fall asleep riding back from the local pub. His own horse knew the way home, but one night, the drunken farmer took the wrong horse from the inn's stables. Unguided, that horse wandered over the cliff, killing them both. Don't drink and drive.
More scientifically, the cliff is a result of displacement on the North Craven Fault, presumably exposed by glaciation.

The field of glacial erratics at Norber (see subsequent images in this set) is to the immediate right of the photo, level with the clifftop.