Where will the rocks be a Zikaron?
Rashi: In Gilgal.
Why does it say here that you will tell your children, and in verse 22, you will inform them?
Malbim: Here, you and the children know the matter (of the Yarden splitting), just you must tell them that it was due to the Aron. It split when the Aron passed! For future generations, it is a witness and remembrance of the miracle itself.