It’s the story of a family who joins a wagon train to travel to California. In one of the episodes, they meet up with another wagon train before they cross the Sierra Mountains and it turns out that other group was the Donner Party. They went their separate ways. (It didn’t mention what happened to the Donners.) Another vivid part of the book had them crossing the desert, they’re almost collapsing from thirst and tormenting mirages of water appear on the horizon. Also, I seem to remember them carving on rocks, joining the marks of others who had traveled that way.