The rocks ranged in size between "fist-sized" and 12 inches in diameter, park officials said. Park officials estimated the debris from the rock fall would have filled the bed of a pickup truck, according to the Salt Lake Tribune.
An ambulance that responded was unable to airlift the girl because of her unstable condition, park officials told CNN. She died en route to an area hospital, according to park officials.
The two adults in the vehicle suffered significant bruising and were taken to an area hospital, the Great Falls Tribune reported. The other two children in the vehicle suffered minor injuries, the newspaper reported.
Officials with the National Park Service said the last time a rockfall caused a death on Going-to-the-Sun Road was 1996, according to the Salt Lake Tribune.