Detroit vs Rock City

Detroit

name
  • The largest city and former capital of Michigan, United States, a major port on the Detroit River, known as the traditional automotive center of the U.S. It is the county seat of Wayne County. 

  • The United States' automotive industry. 

Rock City

name
  • A nickname for Detroit. 

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