hogback vs monadnock

hogback

noun
  • A sharp steep-sided ridge formed by the erosion of tilting strata 

  • A Viking grave marker taking the form of a recumbent monument, generally with a curved (hogbacked) ridge and outwardly curved sides. 

  • A hogframe. 

monadnock

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

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