embankment vs tumulus

embankment

noun
  • a long mound of earth, stone, or similar material, usually built for purposes such as to hold back or store water, for protection from weather or enemies, or to support a road or railway. 

tumulus

noun
  • A mound of earth, especially one placed over a prehistoric tomb; a barrow. 

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