In reply to Dogwatch:
Ian West is THE man for Purbeck geology have a look at http://www.southampton.ac.uk/~imw/
You are right in thinking the rock below the "fault line " is different, it was deposited earlier, and in different sea conditions.
The "faultline" is a band of deposition (mostly sea shells) that is softer than the layers above and below.
The faultline doesn't drop as you move west in a general sense; it is much higher in the Ruckle, and even higher at St Aldhems Head. The differences are due to local faulting, some minor (well seen at the entrance to Benny) and some major (Buttery Corner).