wordwell, the real idea behind a ha-ha is to give the landholder the illusion of unbroken vistas while still keeping out the neighbor's sheep. (see illustration) the definition "sunk fence" is a bit misleading, since
it is only 'sunken' from the one side and wouldn't keep out a field mouse were it not for the ditch.

http://home.mn.rr.com/wwftd/