boulder vs rubble

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. 

rubble

noun
  • A mass or stratum of fragments of rock lying under the alluvium and derived from the neighbouring rock. 

  • The broken remains of an object, usually rock or masonry. 

  • The whole of the bran of wheat before it is sorted into pollard, bran, etc.. 

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