Those twelve stones did Joshua set up in Gilgal - Joshua 4:20
Not content with pitching a cairn of stones on the river's bank, Joshua, at God's command, set up twelve stones in the midst of Jordan, in the place where the feet of the priests that bare the Ark of the Covenant stood. And often, as he came hack to Gilgal, he must have gone out by himself to walk and muse beside the river, turning the outward and the inner gaze to the spot where beneath the flow of the current those stones lay hidden. They were a perpetual memorial of where the people had been, of the grace which had brought them forth, and of the position to which God had conducted them. Children in after days would gather round those mighty boulders and be instructed, and it is a great matter that the deliverances of God should be graven as with a pen of iron on the soft and yielding surface of the child's heart; thus the coming generation shall revere and love the name of Jehovah.
The story of these stones is told again by the Apostle Paul in Ephesians 2. We were dead in trespasses and sins, and lay hopelessly in the grave, like stones in the heart of the river of death. But we were brought forth by God's mighty hand and outstretched arm. We were raised up together with Christ. The resurrection of Jesus is the memorial stone of our position in the sight of God; from this we should never recede. How those old stones would have cried out, if Israel had gone back over the Jordan! And does not Christ's empty grave protest against our living amid the pleasures and cares of the world from which He has gone, and going, has taken us also? This is not our rest; let us make good our standing in the risen Christ.