Faithful, by Janet Fox

Summary: Sixteen-year-old Maggie Bennet’s life is in tatters. Her mother has disappeared, and is presumed dead. The next thing she knows, her father has dragged Maggie away from their elegant Newport home, off on some mad excursion to Yellowstone in Montana.  When she arrives, she finds herself drawn to the frustratingly stubborn, handsome Tom Rowland, the son of a park geologist, and to the wild romantic beauty of Yellowstone itself, just becoming a public park.  Set in its historical era, we see her dilemma in choosing between who she is and who she wants to be.