couloir vs heugh

couloir

noun
  • A steep gorge along a mountainside. 

  • A corridor or passage. 

heugh

noun
  • A steep crag or cliff, especially one with overhanging sides 

  • A glen with steep, overhanging sides 

  • A steep excavation, especially a coal pit 

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