boulder vs crag

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. 

crag

noun
  • A rough, broken fragment of rock. 

  • A rocky outcrop; a rugged steep cliff or rock. 

  • A partially compacted bed of gravel mixed with shells, of the Pliocene to Pleistocene epochs. 

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