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.
Ghost Town Treasure by Clyde Robert Beulla?
https://www.goodreads.com/book/show/1636745.Ghost_Town_Treasure?from_search=true&from_srp=true&qid=eU5Dvt19Uy&rank=1
I think Ghost Town Treasure too. There’s something about an old letter, partial or smudge, and kids reading “gold in the cave” when the writer had written only “cold in the cave.”
Mystery of the Haunted Mine aka Haunted Treasure of the Espectros by Gordon D. Shirreffs.
I also believe this is Ghost Town Treasure. Checked my copy and the diary said the cave was in a canyon beneath the Great Cross. The kids accidentally find this when searching the canyon just before dark – the evening shadows of a ridge and shelf of rock make two lines in the shape of a cross. One boy makes a mark on a tree so they can find the spot during the day. The story takes place in the west in a town named Gold Rock, California.