gravestone vs megalith

gravestone

noun
  • A stone slab set at the head of a grave. 

megalith

noun
  • 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. 

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

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