drumlin vs whaleback

drumlin

noun
  • An elongated hill or ridge of glacial drift. 

whaleback

noun
  • A landform (typically a sand dune) having the form of the back of a whale. 

  • A kind of cargo steamship with a hull that continuously curves above the waterline from vertical to horizontal. 

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