boulder vs shelf

boulder

noun
  • A large marble, in children's games. 

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

  • A session of bouldering; involvement in bouldering. 

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

verb
  • To engage in bouldering. 

shelf

noun
  • A projecting ledge that resembles such an object. 

  • A reef, shoal or sandbar. 

  • The part of a repository where shelvesets are stored. 

  • A flat, rigid structure, fixed at right angles to a wall or forming a part of a cabinet, desk etc., and used to support, store or display objects. 

  • The capacity of such an object 

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