The Collaborative International Dictionary of English v.0.48:

Southdown \South"down`\, a.
   Of or pertaining to the South Downs, a range of pasture hills
   south of the Thames, in England.
   [1913 Webster]

   Southdown sheep (Zool.), a celebrated breed of shortwooled,
      hornless sheep, highly valued on account of the delicacy
      of their flesh. So called from the South Downs where the
      breed originated.
      [1913 Webster]