henge vs megalith

henge

noun
  • A prehistoric enclosure in the form of a circle or circular arc defined by a raised circular bank and a circular ditch usually running inside the bank, with one or more entrances leading into the enclosed open space. 

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 henge and megalith occurred in a corpus of books? (source: Google Ngram Viewer )