hummock vs monadnock

hummock

noun
  • A small hill; a hillock; a knoll. 

  • A fistful. 

  • A ridge or hill of ice in an ice field. 

monadnock

noun
  • A hill or mountain standing isolated above a predominantly flat plain. 

How often have the words hummock and monadnock occurred in a corpus of books? (source: Google Ngram Viewer )