What rocks did Yehoshua put in the Yarden at the Kohanim's feet?
Rashi: These were other rocks.
Why did Yehoshua put rocks at the Kohanim's feet?
Radak: Since Yehoshua did so, surely Hashem commanded him to do so.
Malbim: After they took 12 rocks from where the Kohanim stood, they put these in place of them. They remained as a remembrance of the place of the miracle.
Did he literally put the rocks under the Kohanim's feet?
Radak: He could not, for they did not move until later! Rather, he put the rocks by their feet.