glimpse vs scratch the surface

glimpse

verb
  • To see or view briefly or incompletely. 

  • To appear by glimpses. 

noun
  • A brief look, glance, or peek. 

  • A sudden flash. 

  • A faint idea; an inkling. 

scratch the surface

verb
  • To barely begin; to see or do only a fraction of what is possible. 

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