ice cap vs ice sheet

ice cap

noun
  • A permanent expanse of ice encompassing a large geographical area, e.g. in Earth's polar zones or at high elevation. 

  • An ice pack designed to be worn on the head. 

ice sheet

noun
  • A broad glacial mass with a relatively flat surface. 

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