megalith vs rockpile

megalith

noun
  • A prehistoric monument made up of one or more large stones. 

  • A large stone slab making up a prehistoric monument, or part of such a monument. 

  • A large stone or block of stone used in the construction of a modern structure. 

rockpile

noun
  • A pile of rocks. 

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