The Collaborative International Dictionary of English v.0.48:

Hummock \Hum"mock\, n. [Prob. a dim. of hump. See Hump.]
   1. A rounded knoll or hillock; a rise of ground of no great
      extent, above a level surface.
      [1913 Webster]

   2. A ridge or pile of ice on an ice field.
      [1913 Webster]

   3. Timbered land. See Hammock. [Southern U.S.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

hummock
    n 1: a small natural hill [syn: knoll, mound, hillock,
         hummock, hammock]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "hummock":
   anthill, barrow, brae, butte, down, drumlin, dune, fell, foothills,
   hill, hillock, knob, knoll, molehill, monticle, monticule, moor,
   mound, sand dune, swell