I had a phone call last year from someone in the next village 'Can you come and look at the wall I've found under my garden' !!! I was off like a shot. Beautiful flint walling, a fireplace, a well were eventually revealed in the garden of an old manor house along with ancient broken glass flasks and beautiful pottery, pieces large enough to be identified. Mostly from 17th century. All because they started a new bed where a corner needed tidying up. The previous occupants had been there for years but not quite that long.
re the visited 'yellow garden' the transfer of material from the top to the bottom may have been solifluction effects from the last ice age 10-15,000 years ago when tundra conditions existed beyond the reach of the glaciers. Hereabouts everytime they put in a new road through the chalk the slips show up really well . Summer time partial thaws melted the top layers which have then slid downhill, wherever there was a slope, scouring big channels through the chalk