[1] : The Collaborative International Dictionary of English v.0.48

sunk \sunk\,
   imp.  p. p. of sink.
   1913 webster

   sunk fence, a ditch with a retaining wall, used to divide
      lands without defacing a landscape; a ha-ha.
      1913 webster
see also:
sink sunk fence 

[2] : WordNet (r) 2.0

sunk fence
     n : a ditch with one side being a retaining wall; used to divide
         lands without defacing the landscape syn: ha-ha, haw-haw
see also:
ha-ha haw-haw 

Sunk >> S Words
Sunk, definition of term: Sunk
sunk_pag1.html

Fence >> F Words
Fence, definition of term: Fence
fence_pag1.html

Fence In >> F Words
Fence In, definition of term: Fence In
fence+in_pag1.html

Picket Fence >> P Words
Picket Fence, definition of term: Picket Fence
picket+fence_pag1.html

Ring Fence >> R Words
Ring Fence, definition of term: Ring Fence
ring+fence_pag1.html