and command them, saying, ‘Take up from here, from the midst of the Jordan, from the place where the feet of the priests are standing firm, 12 stones, and make them cross over with you, and put them down in the camp where you will camp tonight.’” Bible

“and make them cross over with you.” The stones were taken from the edge of the Jordan River where the feet of the priests stood in the water, so just as Yahweh was making the Israelites cross the Jordan, the Israelites were making the stones cross over the Jordan. It is as if the stones also were obeying God and going into the Promised Land. The fact that the stones themselves came from the other side of the Jordan adds to the effect of them being a witness of the crossing.

Commentary for: Joshua 4:3