promontory vs sea stack

promontory

noun
  • A high point of land extending into a body of water, headland; cliff. 

  • A rounded elevation in the tympanum of the ear. 

  • A projection on the sacrum. 

sea stack

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

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