morass vs muskeg

morass

noun
  • A tract of soft, wet ground; a marsh; a fen. 

  • Anything that entraps or makes progress difficult. 

muskeg

noun
  • A terrain composed of peat bog with tussocky meadow and woody vegetation including spruce. 

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