then you shall tell them, Because the waters of the Jordan were cut off before the ark of the covenant of Yahweh; when it passed over the Jordan, the waters of the Jordan were cut off: and these stones shall be for a memorial to the children of Israel forever.
"Forever  ..." Now, that cairn is invisible; but the institutions the two cairns (one in the midst of the river, and the other at Gilgal) typified, namely, Christian baptism and the Lord's Supper, shall indeed last forever!  The instructions here show clearly that the "crossing" is still under way, and that the priests were still standing in the midst of the Jordan.  Blair and others have supposed that the Crossing was indeed complete, and that these twelve special representatives of Israel went back to the place where the priests had stood to gather the stones.  It is very difficult to reconstruct all of the action precisely.  In any case, the purpose of the twelve and the memorial was to show the participation of all of the twelve tribes of Israel in this event.