a "ha-ha":
It's a sort of 'invisible wall', designed to keep livestock away from your stately home or ornamental garden; without the visual intrusion of a traditional fence between the lawn and the grazing. It works, effectively, as a small cliff; or a shear fault.

The ground is level, suddenly drops a few feet, then carries on at a lower level. Note that this arrangement is only inconspicuous when viewed from the 'posh' side, looking from the 'plebian' side it's quite visible.

en.wikipedia.org/wiki/Ha-ha for better detail
(The Incredible Sulk Finally remembered his password on, Sun 26 Jul 2009, 1:03, 1 reply)

and from the top of the Wikipedia page
"Ha-ha
For other uses, see Haha.
Not to be confused with laughter."

Lolz.
( Sextant goes to him is emotionally erect, on this day, Sun 26 Jul 2009, 2:40, closed)