carr vs fen

carr

noun
  • A bog or marsh; marshy ground, swampland. 

  • rock 

  • A marsh or fen on which low trees or bushes grow; a marshy woodland. 

fen

noun
  • A type of wetland fed by ground water and runoff, containing peat below the waterline, characteristically alkaline. 

  • A unit of currency in China, one-hundredth of a yuan. 

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