Simplify the following text for a Grade 11 native English-speaking person.
Original: The Grampian Mountains or Grampians ( Am Monadh in Gaelic ) are one of the three major mountain ranges in Scotland , occupying a considerable portion of the Scottish Highlands in northeast Scotland .
Simplified: The Grampian Mountains or Grampians are one of the three major mountain ranges in Scotland , occupying a considerable portion of the Scottish Highlands in northeast Scotland .