crag vs sea stack

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. 

sea stack

noun
  • A pillar of rock that rises from the ocean, formed by surrounding softer ground eroding away. 

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