passage: Imagine you are standing in a farm field in central Illinois .
  The land is so flat you can see for miles and miles .
  On a clear day , you might see a grain silo 20 miles away .
  You might think to yourself , it sure is flat around here .
  If you drive one hundred miles to the south , the landscape changes .
  In southern Illinois , there are rolling hills .
  Why do you think this is ?
  What could have caused these features ?
  There are no big rivers that may have eroded and deposited this material .
  The ground is capable of supporting grass and trees , so wind erosion would not explain it .
  To answer the question , you need to go back 12,000 years .
  Around 12,000 years ago , a giant ice sheet covered much of the Midwest United States .
  Springfield , Illinois , was covered by over a mile of ice .
  Its hard to imagine a mile thick sheet of ice .
  The massive ice sheet , called a glacier , caused the features on the land you see today .
  Where did glaciers go ?
  Where can you see them today ?
  Glaciers are masses of flowing ice .
query_and_answer: How does the geography of central Illinois differ from southern Illinois ? || Central Illinois is flat and Southern Illinois has mountains
evidences: Imagine you are standing in a farm field in central Illinois . The land is so flat you can see for miles and miles . In southern Illinois , there are rolling hills .