back forty vs north forty

back forty

noun
  • The most remote or inaccessible part of any place. 

  • The remote part of a farm. 

north forty

noun
  • Any remote place. 

  • A 40-acre section of land, located in the northernmost part of a farm or ranch. 

  • Farms, ranches, or agricultural land collectively. 

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