delf vs open-pit mine

delf

noun
  • A mine, quarry, pit dug; ditch. 

  • A charge representing a square sod. 

open-pit mine

noun
  • A type of mine where rock or minerals are extracted from the earth by their removal from an open pit or borrow. 

How often have the words delf and open-pit mine occurred in a corpus of books? (source: Google Ngram Viewer )