Joshua 4:9

And Joshua set up twelve stones in the midst of Jordan, where they might become visible when the water was at low stage, in the place where the feet of the priests which bare the Ark of the Covenant stood; and they are there unto this day, to the time when this report was put down or this book written. This second monument was erected by Joshua without special divine direction, but nevertheless with a good purpose, for it served to bring home to the people the fact of God's protection and assistance in the conquest of Canaan.