boulder vs cobblestone

boulder

noun
  • A large mass of stone detached from the surrounding land. 

  • A particle greater than 256 mm in diameter, following the Wentworth scale 

  • A large marble, in children's games. 

  • A session of bouldering; involvement in bouldering. 

verb
  • To engage in bouldering. 

cobblestone

noun
  • The material made from cobblestones. 

  • A rounded stone from a river bed, fit for use as ballast in ships and for paving roads. 

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