It's not on the dig that creates the cliff edge, it's when I back-fill the quarry edge. This occurs when no loss of height has happened in the quarry. Just sometimes when I back-fill it decides it wants to bring the dirt edge up higher than it should be able to and creates a cliff. I've had this happen to the tune of 15 height units just today (and am having to strip-mine just to get rid of it

). I am still experimenting with all of this and will hopefully figure out how to create cliffs on demand.
Also, that screenshot I promised isn't coming tonight. The image uploader isn't working and I can't be bothered to use imgur or whatever
