I am trying to locate a book that I read many times in the late 1970s. (I was in late elementary school.) I probably ordered the book through school and the Scholastic Reader book order program.
The book was a mystery that took place in the west (maybe Arizona?) and the kids solved it when at a certain time of day or year shadows appeared and marked an “x” where the treasure or the entrance to the treasure was located. I seem to recall the words “canyon” “shadow” or “gold” were in the title. It was a paperback with a blue cover and gold writing. It wasn’t a series or Nancy Drew or Hardy Boys.