drumlin vs horseback

drumlin

noun

  • An elongated hill or ridge of glacial drift. 

horseback

noun

  • A ridge of sand, gravel, and boulders. 

  • The back of a horse. 

adv

  • On the back of a horse. 

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