I read this book in about 1962. A girl goes to stay with relatives (I believe) on a farm for the summer. I seem to recall that she was straight laced and the farm family was very laid back. She learned to ride horses and they rode all over the countryside. A particular place I remember them going was to a rocky area (by a creek?) that had Indian paintings or markings on the rocks. I think the climax of the book was them going to possibly a county fair where there was a horse race.