escarpment vs glacis

escarpment

noun
  • A steep descent or declivity; steep face or edge of a ridge; ground about a fortified place, cut away nearly vertically to prevent hostile approach. 

glacis

noun
  • A gentle incline in front of a fortification. 

  • The angled armour plate on the front of a tank; glacis plate. 

  • An erosional or depositional landform, with little slope. 

  • A device for sorting mail which slides parcels across a sloped surface. 

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